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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 machine can insert letters in envelopes at the rate of 120 per minute. Another machine can stamp the envelopes at the rate of 3 per second. How many such stamping machines are needed to keep up with 18 inserting machines of this kind?

(A)12

(B)16

(C)20

(D)22

(E)24

答案和解析

答案：A

解析：

First you can change 1 minute to 60 seconds so that the ratios are both in envelopes per second. One inserting machine inserts letters at the rate of 120 per 60 seconds, or 2 per second. So 18 machines would insert 36 letters per second.

Let x be the number of stamping machines needed to keep up with 18 inserting machines. Then, since one machine stamps 3 envelopes per second, x machines stamp 3 times x envelopes per second. You can write the equation 3 times x = 36, which gives x = 12.