PSAT Math Multiple-Choice Question 644: Answer and Explanation

Question: 644

Linda's 15-gallon car tank has only 2 gallons left when she pulls into a gas station. She wants to purchase only the gas she will need to drive
240 miles and still have 1 gallon remaining. Her car gets 28 miles to the gallon. How many gallons should Linda purchase, to the nearest tenth of a gallon?

• A. 6.6 gallons
• B. 7.6 gallons
• C. 8.6 gallons
• D. 9.6 gallons

Correct Answer: B

Explanation:

(B) If you were told that you had to travel 50 miles and that your car got 10 miles/gallon, you may intuitively see that you need 5 gallons of gas. You get that by dividing 50/10. Following this logic, we can get the number of gallons of gas Linda needs to travel 240 miles by dividing 240 by 28:

240 ÷ 28 = 8.57 gallons

If this doesn't quite make sense, you could also do dimensional analysis to cancel out the units you don't want. You want to cancel out miles and end up with gallons:

Linda also wants to have 1 gallon left, so she'll want to have 9.57 gallons in her tank when she starts out. Linda already has 2 gallons in her tank, so she needs to buy 9.57 - 2 = 7.57 gallons, or 7.6 rounded to the nearest tenth. This matches choice (B).