Integers - GRE Quantitative Reasoning

For each quantity, only count the integers that aren't in the other quantity. Both quantities include the numbers 51 to 98, so those numbers won't affect which is greater. Only Quantity A has 49 and 50 (for a total of 99) and only Quantity B has 99. Since the excluded numbers from both quantities equal 99, you can conclude that the 2 quantities are equal.

The sum of all integers from 1 to 30 can be found using the formula

, where is 30. In this case, the sum equals 465.

When two even numbers are multiplied, they must equal an even number. Also, since both variables are said to be even whole numbers, the answer must fit the requirement that its factors are two even numbers multiplied by one another. The only answer that fits both requirements is which can be factored into the even whole numbers and .

With many questions like this, it might be easier to plug in numbers rather than dealing with theoretical variables. However, given that this question asks for the expression that is not always even or odd but only not necessarily odd, the theoretical route might be our only choice.

Therefore, our best approach is to simply analyze each answer choice.

: Since is odd, is also odd, since and odd number multiplied by an odd number yields an odd product. Since is also odd, multiplying it by will again yield an odd product, so this expression is always odd.

: Since is odd, multiplying it by 2 will yield an even number. Subtracting this number from will also give an odd result, since an odd number minus an even number gives an odd number. Therefore, this answer is also always odd.

: Since both numbers are odd, their product will also always be odd.

: Since is odd, multiplying it by 2 will give an even number. Since is odd, subtracting it from our even number will give an odd number, since an even number minus and odd number is always odd. Therefore, this answer will always be odd.

: Since both numbers are odd, there sum will be even. However, dividing an even number by another even number (2 in our case) does not always produce an even or an odd number. For example, 5 and 7 are both odd. Their sum, 12, is even. Dividing by 2 gives 6, an even number. However, 5 and 9 are also both odd. Their sum, 14, is even, but dividing by 2 gives 7, an odd number. Therefore, this expression isn't necessarily always odd or always even, and is therefore our answer.

To find the answer to this question, calculate the total jelly beans for each person:

Portia: * <Theodore's count of jelly beans>, which is or

Harvey: * <Portia's count of jelly beans>, which is or

So, the total is:

(Do not forget that you need those original for Theodore!)

When two odd whole numbers are multipliedd, they will equal an odd whole number. The only answer that fits the requirements of being an odd and a whole number is since it comes from or .

This question tests basic number properties. Prime numbers are numbers which are divisible only by one and themselves. Answer options '2' and '4' are automatically out, because they will always produce even products with a and b, and the sum of two even products is always even. Since no even number greater than 2 is prime, 2 and 4 cannot be answer options. 3 is tempting, until you remember that the sum of any two multiples of 3 is itself divisible by 3, thereby negating any possible answer for c except 3, which is impossible. There are, however, several possible combinations that work with x = 1. For instance, a = 8 and b = 9 means that 8(1) + 9(1) = 17, which is prime. You only need to find one example to demonstrate that an option works. This eliminates the "None of the other answers" option as well.

Factor each of our values into prime factors:

350 = 2 * 52 * 7

6270= 2 * 3 * 5 * 11 * 19

To find the least common multiple, we must choose the larger exponent for each of the prime factors involved. Therefore, we will select 2, 52, and 7 from 350 and 3, 11, and 19 from 6270.

Therefore, our least common multiple is 2 * 3 * 52 * 7 * 11 * 19 = 219,450.

Each of the numbers on the list must be able to "fit" (multiply evenly) into a larger number at the same time. I.e. the largest number (multiple) divided by any of the listed numbers will result in a whole number. For the coefficients, the maximum value is 10, and multiplying the highest two coefficients give us 60. Since 60 is divisible evenly by the lower values, we know that it is the least common multiple for the list. For the variables, both x and y will fit evenly into a theoretical number, "xy". We do not need an exponential version of this multiple as there are no exponents in the original list.

The least common multiple is the smallest number that is a multiple of all the numbers in the group. Let's list some multiples of the two numbers and find the smallest number in common to both.

multiples of 45: 45, 90, 135, 180, 225, 270, ...

multiples of 60: 60, 120, 180, 240, 300, 360, ...

The smallest number in common is 180.

First, find all of the factors of each number. Factors are the numbers that, like 16 and 24, can evenly be divided. The factors of 16 are 1, 2, 4, 8, 16. The factors of 24 are 1, 2, 3, 4, 6, 8, 12, 24.

Now, to find the greatest common factor, we find the largest number that is on both lists. This number is 8.

To make things easier, note 6930 is divisible by 30:

6930 = 231 * 30 = 3 * 77 * 3 * 2 * 5 = 3 * 7 * 11 * 3 * 2 * 5 = 2 * 32 * 5 * 7 * 11

288 = 2 * 144 = 2 * 12 * 12 = 2 * 2 * 2 * 3 * 2 * 2 * 3 = 25 * 32

Consider each of these "next to each other":

25 * 32

2 * 32 * 5 * 7 * 11

Each shares factors of 2 and 3. In the case of 2, they share 1 factor. In the case of 3, they share 2 factors. Therefore, their greatest common factor is: 2 * 32 = 2 * 9 = 18

The greatest common factor is the greatest factor that divides both numbers. To find the greatest common factor, first list the prime factors of each number.

18 = 2 * 3 * 3

24 = 2 * 2 * 2 * 3

18 and 24 share one 2 and one 3 in common. We multiply them to get the GCF, so 2 * 3 = 6 is the GCF of 18 and 24.

The third smallest prime number is 5. (Don't forget that 2 is a prime number, but 1 is not!)

The smallest two digit prime number is 11.

Now we can evaluate the entire expression:

For starters, 1 is not a prime number, so eliminate the answer choices with 1 in them. Even if you have no idea what twin primes are, at least you've narrowed down the possibilities.

Twin primes are consecutive prime numbers with one even number in between them. 3 and 5 is the only set of twin primes listed. 2 and 3 are not separated by any numbers, and 13 and 19 are not consecutive primes, nor are they separated by one even number only. You should do your best to remember definitions and formulas such as this one, because these questions are considered "free" points on the test. There is no real math involved, just something to remember! Being able to answer a question like this quickly will give you more time for the computationally advanced problems.

As we go up on the number line, the number of primes decreases almost exponentially. Therefore there are far more prime numbers between 0 and 100 than there are between 101 and 200. This is a general number theory point that is important to know, but trying to come up with some primes in these two groups will also quickly demonstrate this principle.

All of these statements are true. Let's go through them.

1. There are no negative prime numbers. This appears as if it might be false, but in fact, the prime numbers are defined as whole numbers greater than one that are divisible by only one and itself.

2. Every number except 0 and 1 is a prime number or product of primes. This is also true. Let's look at the factorization of a number that isn't prime. For example, 6 = 2 * 3, which is a product of primes. 12 = 2 * 2 * 3, which is also a product of primes.

3. Every number has a unique prime factorization. We just saw that every number is either prime, or a product of primes. Therefore each number must have a unique prime factorization. Just as above, 6 is the product of two primes, 2 and 3. No other number can be made by mulitplying 2 * 3. The same is true for 12. When we multiply 2 * 2 * 3, the only number we will ever get is 12.

Let's go through the five statements.

The sum of two primes is always even: This is only true of the odd primes. 2 is also a prime number, however, and 2 plus an odd number is odd.

Every positive prime has a corresponding negative prime: This is also false. There are no negative primes. A prime number is defined as a number greater than 1 that is divisible by only 1 and itself.

There are only two primes that are consecutive positive integers on the number line: This is true and therefore the correct answer. 2 and 3 are the only primes that are consecutive. Because 2 is the only even prime, all other primes must have at least one number in between them (since every two odd numbers are separated by an even).

Multiplying two primes will always produce an odd number: This is also only true of odd primes. 2 * odd prime = even.

The distribution of primes is random: False. The primes are logarithmically distributed.

Quantity A: The smallest prime number is 2. We also need the least common multiple of 5 and 10, which is 10.

So Quantity A = 2 * 3 / 10 = 3/5

Quantity B: The smallest odd prime is 3. The second smallest odd prime is 5.

So Quantity B = 3 * 2 / 5 = 6/5

Quantity B is greater.