How to find the greatest or least number of combinations - PSAT Math
Card 1 of 112
There are seven unique placemats around a circular table. How many different orders of placemats are possible?
There are seven unique placemats around a circular table. How many different orders of placemats are possible?
Tap to reveal answer
Since the table is circular, you need to find the total number of orders and divide this number by 7.
The total number of different orders that the placemats could be set in is 7! (7 factorial).
7!/7 = 6! = 720
Note that had this been a linear, and not circular, arrangement there would be no need to divide by 7. But in a circular arrangement there are no "ends" so you must divide by N! by N to account for the circular arrangement.
Since the table is circular, you need to find the total number of orders and divide this number by 7.
The total number of different orders that the placemats could be set in is 7! (7 factorial).
7!/7 = 6! = 720
Note that had this been a linear, and not circular, arrangement there would be no need to divide by 7. But in a circular arrangement there are no "ends" so you must divide by N! by N to account for the circular arrangement.
← Didn't Know|Knew It →
Mark has 5 pants and 7 shirts in his closet. He wants to wear a different pant/shirt combination each day without buying new clothes for as long as he can. How many weeks can he do this for?
Mark has 5 pants and 7 shirts in his closet. He wants to wear a different pant/shirt combination each day without buying new clothes for as long as he can. How many weeks can he do this for?
Tap to reveal answer
The fundamental counting principle says that if you want to determine the number of ways that two independent events can happen, multiply the number of ways each event can happen together. In this case, there are 5 * 7, or 35 unique combinations of pants & shirts Mark can wear. If he wears one combination each day, he can last 35 days, or 5 weeks, without buying new clothes.
The fundamental counting principle says that if you want to determine the number of ways that two independent events can happen, multiply the number of ways each event can happen together. In this case, there are 5 * 7, or 35 unique combinations of pants & shirts Mark can wear. If he wears one combination each day, he can last 35 days, or 5 weeks, without buying new clothes.
← Didn't Know|Knew It →
There are five pictures but only four display cases. The display cases are unique. How many different arrangements of pictures in display cases can be created?
There are five pictures but only four display cases. The display cases are unique. How many different arrangements of pictures in display cases can be created?
Tap to reveal answer
There are five possible choices for the first space. For the second there are four possible, three for the third, and two for the fourth. 5 * 4 * 3 * 2 = 120 possible arrangements.
There are five possible choices for the first space. For the second there are four possible, three for the third, and two for the fourth. 5 * 4 * 3 * 2 = 120 possible arrangements.
← Didn't Know|Knew It →
Twenty students enter a contest at school. The contest offers a first, second, and third prize. How many different combinations of 1st, 2nd, and 3rd place winners can there be?
Twenty students enter a contest at school. The contest offers a first, second, and third prize. How many different combinations of 1st, 2nd, and 3rd place winners can there be?
Tap to reveal answer
This is a permutation problem, because we are looking for the number of groups of winners. Consider the three positions, and how many choices there are for each position: There are 20 choices for 1st place, 19 for 2nd place, and 18 for 3rd place.
20, 19, 18
Multiply to get 6840.
This is a permutation problem, because we are looking for the number of groups of winners. Consider the three positions, and how many choices there are for each position: There are 20 choices for 1st place, 19 for 2nd place, and 18 for 3rd place.
20, 19, 18
Multiply to get 6840.
← Didn't Know|Knew It →
A baker has four different types of frosting, three different kinds of sprinkles, and 8 different cookie cutters. How many different cookie combinations can the baker create if each cookie has one type of frosting and one type of sprinkle?
A baker has four different types of frosting, three different kinds of sprinkles, and 8 different cookie cutters. How many different cookie combinations can the baker create if each cookie has one type of frosting and one type of sprinkle?
Tap to reveal answer
Since this a combination problem and we want to know how many different ways the cookies can be created we can solve this using the Fundamental counting principle. 4 x 3 x 8 = 96
Multiplying each of the possible choices together.
Since this a combination problem and we want to know how many different ways the cookies can be created we can solve this using the Fundamental counting principle. 4 x 3 x 8 = 96
Multiplying each of the possible choices together.
← Didn't Know|Knew It →
If a series of license plates is to be produced that all have the same pattern of three letters followed by three numbers, roughly how many alphanumeric combinations are possible?
If a series of license plates is to be produced that all have the same pattern of three letters followed by three numbers, roughly how many alphanumeric combinations are possible?
Tap to reveal answer
The total number of possible combinations of a series of items is the product of the total possibility for each of the items. Thus, for the letters, there are 26 possibilities for each of the 3 slots, and for the numbers, there are 10 possibilities for each of the 3 slots. The total number of combinations is then: 26 x 26 x 26 x 10 x 10 x 10 = 17,576,000 ≈ 18 million.
The total number of possible combinations of a series of items is the product of the total possibility for each of the items. Thus, for the letters, there are 26 possibilities for each of the 3 slots, and for the numbers, there are 10 possibilities for each of the 3 slots. The total number of combinations is then: 26 x 26 x 26 x 10 x 10 x 10 = 17,576,000 ≈ 18 million.
← Didn't Know|Knew It →
If there are 8 points in a plane, and no 3 of the points lie along the same line, how many unique lines can be drawn between pairs of these 8 points?
If there are 8 points in a plane, and no 3 of the points lie along the same line, how many unique lines can be drawn between pairs of these 8 points?
Tap to reveal answer
The formula for the number of lines determined by n points, no three of which are “collinear” (on the same line), is n(n-1)/2. To find the number of lines determined by 8 points, we use 8 in the formula to find 8(8-1)/2=8(7)/2=56/2=28. (The formula is derived from two facts: the fact that each point forms a line with each other point, hence n(n-1), and the fact that this relationship is symmetric (i.e. if a forms a line with b, then b forms a line with a), hence dividing by 2.)
The formula for the number of lines determined by n points, no three of which are “collinear” (on the same line), is n(n-1)/2. To find the number of lines determined by 8 points, we use 8 in the formula to find 8(8-1)/2=8(7)/2=56/2=28. (The formula is derived from two facts: the fact that each point forms a line with each other point, hence n(n-1), and the fact that this relationship is symmetric (i.e. if a forms a line with b, then b forms a line with a), hence dividing by 2.)
← Didn't Know|Knew It →
8 people locked in a room take turns holding hands with each person only once. How many hand holdings take place?
8 people locked in a room take turns holding hands with each person only once. How many hand holdings take place?
Tap to reveal answer
The first person holds 7 hands. The second holds six by virtue of already having help the first person’s hand. This continues until through all 8 people. 7+6+5+4+3+2+1=28.
The first person holds 7 hands. The second holds six by virtue of already having help the first person’s hand. This continues until through all 8 people. 7+6+5+4+3+2+1=28.
← Didn't Know|Knew It →
At an ice cream store, there are 5 flavors of ice cream: strawberry, vanilla, chocolate, mint, and banana. How many different 3-flavor ice cream cones can be made?
At an ice cream store, there are 5 flavors of ice cream: strawberry, vanilla, chocolate, mint, and banana. How many different 3-flavor ice cream cones can be made?
Tap to reveal answer
There are 5x4x3 ways to arrange 5 flavors in 3 ways. However, in this case, the order of the flavors does not matter (e.g., a cone with strawberry, mint, and banana is the same as a cone with mint, banana, and strawberry). So we have to divide 5x4x3 by the number of ways we can arrange 3 different things which is 3x2x1. So (5x4x3)/(3x2x1) is 10.
One can also use the combination formula for this problem: nCr = n! / (n-r)! r!
Therefore: 5C3 = 5! / 3! 2!
= 10
(Note: an example of a counting problem in which order would matter is a lock or passcode situation. The permutation 3-5-7 for a three number lock or passcode is a distinct outcome from 5-7-3, and thus both must be counted.)
There are 5x4x3 ways to arrange 5 flavors in 3 ways. However, in this case, the order of the flavors does not matter (e.g., a cone with strawberry, mint, and banana is the same as a cone with mint, banana, and strawberry). So we have to divide 5x4x3 by the number of ways we can arrange 3 different things which is 3x2x1. So (5x4x3)/(3x2x1) is 10.
One can also use the combination formula for this problem: nCr = n! / (n-r)! r!
Therefore: 5C3 = 5! / 3! 2!
= 10
(Note: an example of a counting problem in which order would matter is a lock or passcode situation. The permutation 3-5-7 for a three number lock or passcode is a distinct outcome from 5-7-3, and thus both must be counted.)
← Didn't Know|Knew It →
At a deli you can choose from either Italian bread, whole wheat bread, or sourdough bread. You can choose turkey or roast beef as your meat and provolone or mozzarella as your cheese. If you have to choose a bread, a meat, and a cheese, how many possible sandwich combinations can you have?
At a deli you can choose from either Italian bread, whole wheat bread, or sourdough bread. You can choose turkey or roast beef as your meat and provolone or mozzarella as your cheese. If you have to choose a bread, a meat, and a cheese, how many possible sandwich combinations can you have?
Tap to reveal answer
You have 3 possible types of bread, 2 possible types of meat, and 2 possible types of cheese. Multiplying them out you get 3*2*2, giving you 12 possible combinations.
You have 3 possible types of bread, 2 possible types of meat, and 2 possible types of cheese. Multiplying them out you get 3*2*2, giving you 12 possible combinations.
← Didn't Know|Knew It →
Shannon decided to go to nearby café for lunch. She can have a sandwich made on either wheat or white bread. The café offers cheddar, Swiss, and American for cheese choices. For meat, Shannon can choose ham, turkey, bologna, roast beef, or salami. How many cheese and meat sandwich options does Shannon have to choose from?
Shannon decided to go to nearby café for lunch. She can have a sandwich made on either wheat or white bread. The café offers cheddar, Swiss, and American for cheese choices. For meat, Shannon can choose ham, turkey, bologna, roast beef, or salami. How many cheese and meat sandwich options does Shannon have to choose from?
Tap to reveal answer
2 bread choices * 3 cheese choices * 5 meat choices = 30 sandwich choices
2 bread choices * 3 cheese choices * 5 meat choices = 30 sandwich choices
← Didn't Know|Knew It →
An ice cream parlor serves 36 ice cream flavors. You can order any flavor in a small, medium or large and can choose between a waffle cone and a cup. How many possible combinations could you possibly order?
An ice cream parlor serves 36 ice cream flavors. You can order any flavor in a small, medium or large and can choose between a waffle cone and a cup. How many possible combinations could you possibly order?
Tap to reveal answer
36 possible flavors * 3 possible sizes * 2 possible cones = 216 possible combinations.
36 possible flavors * 3 possible sizes * 2 possible cones = 216 possible combinations.
← Didn't Know|Knew It →
For a certain lunch special, customers must order a salad, an entree, and a dessert. If there are three different salads, four different entrees, and two different desserts available, then how many different lunch specials are possible?
For a certain lunch special, customers must order a salad, an entree, and a dessert. If there are three different salads, four different entrees, and two different desserts available, then how many different lunch specials are possible?
Tap to reveal answer
Customers must choose a salad, an entree, and a dessert. There are three different salads, four entrees, and two desserts.
The simplest way of determining the number of combinations is by multiplying the number of options for each part of the meal. In other words, we can find the product of 3, 4, and 2, which would give us 24.
Sometimes, if you can't think of a way to mathetimatically determine all of the different combinations of something, it helps to write out as many as you can. Let's write out all of the possible cominbations just to verify that there are 24. Let's call the different salads _S_1, _S_2, and _S_3. We will call the four entrees _E_1, _E_2, _E_3, and _E_4, and we will call the desserts _D_1 and _D_2.
Here are the possible lunch special combinations:
_S_1, _E_1, _D_1
_S_1, _E_1, _D_2
_S_1, _E_2, _D_1
_S_1, _E_2, _D_2
_S_1, _E_3, _D_1
_S_1, _E_3, _D_2
_S_1, _E_4, _D_1
_S_1, _E_4, _D_2
_S_2, _E_1, _D_1
_S_2, _E_1, _D_2
_S_2, _E_2, _D_1
_S_2, _E_2, _D_2
_S_2, _E_3, _D_1
_S_2, _E_3, _D_2
_S_2, _E_4, _D_1
_S_2, _E_4, _D_2
_S_3, _E_1, _D_1
_S_3, _E_1, _D_2
_S_3, _E_2, _D_1
_S_3, _E_2, _D_2
_S_3, _E_3, _D_1
_S_3, _E_3, _D_2
_S_3, _E_4, _D_1
_S_3, _E_4, _D_2
The answer is 24.
Customers must choose a salad, an entree, and a dessert. There are three different salads, four entrees, and two desserts.
The simplest way of determining the number of combinations is by multiplying the number of options for each part of the meal. In other words, we can find the product of 3, 4, and 2, which would give us 24.
Sometimes, if you can't think of a way to mathetimatically determine all of the different combinations of something, it helps to write out as many as you can. Let's write out all of the possible cominbations just to verify that there are 24. Let's call the different salads _S_1, _S_2, and _S_3. We will call the four entrees _E_1, _E_2, _E_3, and _E_4, and we will call the desserts _D_1 and _D_2.
Here are the possible lunch special combinations:
_S_1, _E_1, _D_1
_S_1, _E_1, _D_2
_S_1, _E_2, _D_1
_S_1, _E_2, _D_2
_S_1, _E_3, _D_1
_S_1, _E_3, _D_2
_S_1, _E_4, _D_1
_S_1, _E_4, _D_2
_S_2, _E_1, _D_1
_S_2, _E_1, _D_2
_S_2, _E_2, _D_1
_S_2, _E_2, _D_2
_S_2, _E_3, _D_1
_S_2, _E_3, _D_2
_S_2, _E_4, _D_1
_S_2, _E_4, _D_2
_S_3, _E_1, _D_1
_S_3, _E_1, _D_2
_S_3, _E_2, _D_1
_S_3, _E_2, _D_2
_S_3, _E_3, _D_1
_S_3, _E_3, _D_2
_S_3, _E_4, _D_1
_S_3, _E_4, _D_2
The answer is 24.
← Didn't Know|Knew It →
Tap to reveal answer
← Didn't Know|Knew It →
A group of friends decide to go out to the movies. Fred and Tom are bringing dates, while their 2 friends are going alone. When the friends arrive at the movie theater, they find a row of six seats so they can all sit together.
If Fred and Tom must sit next to their dates, how many different ways can the group sit down?
A group of friends decide to go out to the movies. Fred and Tom are bringing dates, while their 2 friends are going alone. When the friends arrive at the movie theater, they find a row of six seats so they can all sit together.
If Fred and Tom must sit next to their dates, how many different ways can the group sit down?
Tap to reveal answer
Think of the seats as an arrangement of people in a line. Fred and Tom must sit next to their dates, so you can treat the pair as a single object. The only difference is that we must then multiply by 2, since we can switch the order in which they sit down at will (either Fred or his date can sit on the left).
So instead of dealing with 6 objects, we now simply work with 4. An arrangement of 4 objects, can be made in 
different ways. You can choose any of 4 objects to be in the first spot. Once that spot is taken, you move onto the next of four spots. You place any of the remaining three there, giving you 3 more choices (or multiplying by 3). You do the same thing 2 more times to end up with 24 possibilities.
Finally, you have to take into account switching the positions of Fred/Tom and their respective dates. Since there are two pairs, you multiply by 2 twice. This gives you
different arrangements.
Think of the seats as an arrangement of people in a line. Fred and Tom must sit next to their dates, so you can treat the pair as a single object. The only difference is that we must then multiply by 2, since we can switch the order in which they sit down at will (either Fred or his date can sit on the left).
So instead of dealing with 6 objects, we now simply work with 4. An arrangement of 4 objects, can be made in
Finally, you have to take into account switching the positions of Fred/Tom and their respective dates. Since there are two pairs, you multiply by 2 twice. This gives you
← Didn't Know|Knew It →
The art club must choose a leadership committee of 3 students. If any
member can be on the committee, how many different combnations of 3
students can be selected from the 15 members of the art club?
The art club must choose a leadership committee of 3 students. If any
member can be on the committee, how many different combnations of 3
students can be selected from the 15 members of the art club?
Tap to reveal answer
← Didn't Know|Knew It →
There are seven unique placemats around a circular table. How many different orders of placemats are possible?
There are seven unique placemats around a circular table. How many different orders of placemats are possible?
Tap to reveal answer
Since the table is circular, you need to find the total number of orders and divide this number by 7.
The total number of different orders that the placemats could be set in is 7! (7 factorial).
7!/7 = 6! = 720
Note that had this been a linear, and not circular, arrangement there would be no need to divide by 7. But in a circular arrangement there are no "ends" so you must divide by N! by N to account for the circular arrangement.
Since the table is circular, you need to find the total number of orders and divide this number by 7.
The total number of different orders that the placemats could be set in is 7! (7 factorial).
7!/7 = 6! = 720
Note that had this been a linear, and not circular, arrangement there would be no need to divide by 7. But in a circular arrangement there are no "ends" so you must divide by N! by N to account for the circular arrangement.
← Didn't Know|Knew It →
Mark has 5 pants and 7 shirts in his closet. He wants to wear a different pant/shirt combination each day without buying new clothes for as long as he can. How many weeks can he do this for?
Mark has 5 pants and 7 shirts in his closet. He wants to wear a different pant/shirt combination each day without buying new clothes for as long as he can. How many weeks can he do this for?
Tap to reveal answer
The fundamental counting principle says that if you want to determine the number of ways that two independent events can happen, multiply the number of ways each event can happen together. In this case, there are 5 * 7, or 35 unique combinations of pants & shirts Mark can wear. If he wears one combination each day, he can last 35 days, or 5 weeks, without buying new clothes.
The fundamental counting principle says that if you want to determine the number of ways that two independent events can happen, multiply the number of ways each event can happen together. In this case, there are 5 * 7, or 35 unique combinations of pants & shirts Mark can wear. If he wears one combination each day, he can last 35 days, or 5 weeks, without buying new clothes.
← Didn't Know|Knew It →
There are five pictures but only four display cases. The display cases are unique. How many different arrangements of pictures in display cases can be created?
There are five pictures but only four display cases. The display cases are unique. How many different arrangements of pictures in display cases can be created?
Tap to reveal answer
There are five possible choices for the first space. For the second there are four possible, three for the third, and two for the fourth. 5 * 4 * 3 * 2 = 120 possible arrangements.
There are five possible choices for the first space. For the second there are four possible, three for the third, and two for the fourth. 5 * 4 * 3 * 2 = 120 possible arrangements.
← Didn't Know|Knew It →
Twenty students enter a contest at school. The contest offers a first, second, and third prize. How many different combinations of 1st, 2nd, and 3rd place winners can there be?
Twenty students enter a contest at school. The contest offers a first, second, and third prize. How many different combinations of 1st, 2nd, and 3rd place winners can there be?
Tap to reveal answer
This is a permutation problem, because we are looking for the number of groups of winners. Consider the three positions, and how many choices there are for each position: There are 20 choices for 1st place, 19 for 2nd place, and 18 for 3rd place.
20, 19, 18
Multiply to get 6840.
This is a permutation problem, because we are looking for the number of groups of winners. Consider the three positions, and how many choices there are for each position: There are 20 choices for 1st place, 19 for 2nd place, and 18 for 3rd place.
20, 19, 18
Multiply to get 6840.
← Didn't Know|Knew It →