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machine

by shibal » Sun Jul 12, 2009 3:33 pm
Two machine types, Type R and Type S, operate at a constant rate. R does a job in 36 hours. S does same job in 18. If we use the same number of each type to do the job in 2 hours, how many Type r machines do we need?

oa 6
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by ssmiles08 » Sun Jul 12, 2009 3:54 pm
First, find the combined rate of both machines:

1/36 + 1/18 = 3/36 = 1/12.

Both of them work at a rate of 1/12.

which means 1 job takes 12 hours to complete for both machines working together.

in 2 hours, 1/6 of a job would be done if there were 1 R and 1 S.

since there are equal machines of R and S, 6 machines would work at a rate of 1/6 to get the job done in 2 hours.

so the number of machines is 6.
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