SAT Math Multiple Choice Practice Question 591: Answer and Explanation

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Question: 591

5. If y is directly proportional to the square of x, then which of the following tables could represent values of x and y ?


Correct Answer: D


D The term direct proportion means that once you square x, the ratio between x2 and y should be the same for all pairs in the table. Use your test booklet as scratch paper and write the value for x2 for each pair to check the ratios. In A, 22 = 4, for a ratio of 1 : 4. This does not match the 9 : 6 ratio of the next pair, so eliminate A. The first ratio in B is 1 : 2, but the second is 16 : 8, so eliminate B. In C the first ratio is 4 : 3, but the second is 16 : 9, so eliminate C. Only D is a match: 1 : 3, 4 : 12, and 9 : 27 are all equivalent ratios.

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