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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 per second. How many such stamping machines are needed to keep up with inserting machines of this kind?

Right! (Applause.)

Explanation

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

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