Ratios and Proportional Relationships: Computing Unit Rates with Fractions (CCSS.7.RP.1)

Divide total cost by number of notebooks: $56 \div 8 = 7$ dollars per notebook. Choice A does not divide. Choice C comes from dividing by the wrong number (like $56 \div 7$). Choice D flips the rate (notebooks per dollar) instead of dollars per notebook.

Divide cost by quantity: $48 \div 12 = 4$ dollars per notebook. 0.25 dollars per notebook flips the division ($12 \div 48$). 48 dollars per notebook shows no division. 12 dollars per notebook comes from dividing by 4 instead of 12 or miscalculating.

Divide words by minutes: 600 ÷ 5 = 120 words/minute. B divides the wrong way. C is not per 1 minute (not a unit rate). D flips to minutes per word.

Divide words by minutes: $900 \div 15 = 60$ words per minute. 15 words/minute confuses the roles of words and minutes. 900/15 words/minute is not simplified to a unit rate. 0.0167 words/minute comes from $15 \div 900$, which is the reverse of what we need.

Divide distance by time: 180 ÷ 4 = 45, so 45 miles/hour. '180 miles per 4 hours' is not reduced to per 1 hour, '4 hours per 180 miles' flips the units, and '1 hour per 45 miles' gives hours per mile, not miles per hour.

Compute miles ÷ hours: $180 \div 3 = 60$ miles per hour. 180 miles per hour shows no division. 3 miles per hour comes from dividing hours by miles ($3 \div 180$). 540 miles per hour comes from multiplying instead of dividing ($180 \times 3$).

Divide pages by hours: $180 \div 3 = 60$, so 60 pages/hour. A does not divide at all. B flips the units to hours per page. D divides the wrong way ($3 \div 180$).

Divide pages by hours: 210 ÷ 7 = 30 pages/hour. A does not divide. C divides the wrong way. D flips the units to hours per page.

Divide area by time: $600 \div 4 = 150$ square feet per hour. 2400 square feet/hour multiplies instead of dividing. 600/4 square feet/hour is not simplified to a unit rate. 0.0067 square feet/hour comes from $4 \div 600$, which reverses the order.

Divide cost by quantity: $25 \div 5 = 5$ dollars per notebook. 25 dollars per notebook forgets to divide. 25/5 dollars per notebook is not simplified to a unit rate. 5/25 dollars per notebook flips the order ($5 \div 25$), giving the wrong unit rate.