GMATPrep Test 1 : PS #21

[kaizenwerks]

Good afternoon,

I was wondering if someone could explain this question to me. Thanks!


Each of 10 machines works at the same constant rate doing a certain job. The amount of time needed by the 10 machines, working together, to complete the job is 16 hours. How many hours would be needed if only 8 of the machines, working together, were used to complete the job?

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[ri2007]

Ans should be 20

Assume one machine working alone can complete the job in X hours
so 10 machines together will complete 10/x in one hour.
now it is give 10 machines working together complete the job in 16 hrs.
so (10/x) = 1/16
so X = 10 * 16
so 8 machines working together will complete 8/10*16 in one hour = 1/20
so they will need 20 hrs to complete the job

Pls confirm the ans.

[kaizenwerks]

Yes the answer is correct. Thanks again for your help and prompt response!

[gmatguy16]

the number of machine hours should stay the same in both cases ,in first case machine hours=10*16=160 ....so now since we have only 8 machines,we need 160/8=20 hrs per machine....