考试类型: 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

正确答案: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 3x envelopes per second. You can write the equation 3x=36, which gives x=12.