PSAT Math Multiple-Choice Question 31: Answer and Explanation

Question: 31

David acquired a data plan for his smart phone. Every month he pays a flat rate of $25 and an additional $0.05 for every megabyte he goes over his monthly limit. Which of the following represents David's monthly data bill when he goes over his limit by m megabytes?

• A. 25 + 1.05m
• B. 25 + 0.05m
• C. 0.05(25 + m)
• D. 1.05(25 + m)

Correct Answer: B

Explanation:

B

The question asks for an expression that models a specific situation. There are variables in the answer choices, so plug in. Make m = 10. If David goes over his limit by 10 megabytes, then David pays $25 + $0.05(10) = $25.50. This is the target value; circle it. Now plug m = 10 into the answer choices to see which one matches the target value. Choice (A) becomes 25 + 1.05(10) = $35.50. This does not match the target, so eliminate (A). Choice (B) becomes 25 + 0.05(10) = $25.50. Keep (B), but check the remaining answers just in case. Choice (C) becomes 0.05(25 + 10) = $1.75. Eliminate (C). Choice (D) becomes 1.05(25 + 10) = $36.75. Eliminate (D). The correct answer is (B).