gmatmba wrote:

Machine A and Machine B are each used to manufacture 660 sprockets. It takes machine A 10 hours longer to produce 660 sprockets than machine B. Machine B produces 10% more sprockets per hour than machine A. How many sprockets per hour does machine A produce

A. 6

B. 6.6

C. 60

D. 100

E. 110

This took me a long time to calculate and figure out. Does anyone have a quick method that you can show me?

Source suggested backsolving due to time constraints.

Rate A : Rate B = 10: 11 (because machine B produces 10% more sprockets per hour. So if machine A produces 10 sprockets per hr, machine B produces 11 sprockets per hour)

Time A : Time B = 11 : 10 (Ratio of time taken by machine A and B is inverse of rate)

Now, given that it takes machine A 10 hours longer to produce 660 sprockets than machine B. So actual difference between the time taken is 10 hrs for 660 sprockets. Since difference on the ratio scale is 1, multiplier is 10.

Machine A takes 110 hrs to produce 660 sprockets and machine B takes 100 hrs to produce 600 sprockets.

So machine A makes 660/110 = 6 sprockets per hour.

For more on ratios, check:

http://www.veritasprep.com/blog/2011/03 ... of-ratios/
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