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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 
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?
Hint
First change 
minute to 
seconds so that the ratios are both in envelopes per second. Then find out how many envelopes per second are filled by 
inserting machines. Let 
be the number of stamping machines, working at the rate of 
envelopes per second, needed to stamp that number of envelopes. Since the stamping machines have to stamp the same number of envelopes per second that the inserting machines put letters in, it will be possible to write an equation and solve for 
.
minute to 
seconds so that the ratios are both in envelopes per second. One inserting machine inserts letters at the rate of 
per 
seconds, or 
per second. So 
machines would insert 
letters per second.
be the number of stamping machines needed to keep up with 
inserting machines. Then, since one machine stamps 
envelopes per second, 
machines stamp 
envelopes per second. You can write the equation 
, which gives 
.