Question

A and B take 12 hours to do a job, B and C take 15 hours, and C and A take 20 hours. How much time do they take to complete the task together?

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Answer 1

Answer:In 1 hour A can complete 1/20 part. In 1 hour B can complete 1/5 part. Let in 1 hour C can complete 1/c part. So jointly A & C can complete (1/20+1/c) part and which is 1/10. So 1/10+1/c=1/10, or 1/c=1/10-1/20=1/20 part. Now jointly A,B & C can complete 1/20+1/5+1/20=1/10+1/5=3/10 part in 1 hour. So they will take 1÷3/10=10/3 hrs to complete the work, which is 10×60/3=200 mins=3hrs.20mins.

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