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Mathematics>Standard Multiple Choice

Read the following SAT test question and then click on a button to select your answer.

If a-b = b-3 = 6, what is the value of a?

(A) -3
(B) 6
(C) 9
(D) 15
(E) 18

答案和解析

答案：D

解析：

Both a-b = 6 and b-3 = 6. The equation a-b = 6 cannot be solved for a because there are two unknowns. The value of b can be found by solving the equation b-3 = 6 for b. It follows that b must equal 9. The value of b can now be substituted into the equation a-b = 6, giving a-9 = 6. Therefore, a must equal 15.