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Mathematics>Standard Multiple Choice

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

A geologist has 10 rocks of equal weight. If 6 rocks and a 10-ounce weight balance on a scale with 4 rocks and a 22-ounce weight, what is the weight, in ounces, of one of these rocks?

(A) 4
(B) 5
(C) 6
(D) 7
(E) 8

答案和解析

答案：C

解析：

Let r equal the weight of each rock, in ounces. The weight on one side of the scale is (6r) + 10 ounces and the weight on the other side is (4r) plus 22 ounces. Since the two weights balance, you can write the equation (6r) +10 = (4r) + 22. Solving this equation yields r equals 6.