Machine A can finish a job in 10 hours and machine B can finish it in 15 hours. How many hours do they need working together?

Answer and explanation

Correct answer: A. 6

  1. Step 1: In one hour A does 1/10 of the job and B does 1/15.
  2. Step 2: Together they do 1/10 + 1/15 = 25/150 of the job per hour, so the job takes 150/25 = 6 hours.
Answer
6.
Why not the others: Averaging or adding the two times ignores that the machines work at the same time; you must add their hourly rates and invert.

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