PSAT Math Multiple-Choice Question 362: Answer and Explanation

Question: 362

The number of cars that can safely pass through a stoplight is directly proportional to the length of time in seconds that the light is green. If 9 cars can safely pass through a light that stays green for 36 seconds, how many cars can safely pass through a light that stays green for 24 seconds?

• A. 4
• B. 6
• C. 7
• D. 8

Correct Answer: B

Explanation:

B

Difficulty: Easy

Getting to the Answer: To answer a question that says "directly proportional," set two ratios equal to each other and solve for the missing amount. Don't forget—match the units in the numerators and in the denominators on both sides.

Let c equal the number of cars that can safely pass through a light that lasts 24 seconds. Set up the proportion and solve:

Therefore, (B) is correct.