Machine A can finish a job in 6 hours and machine B can finish it in 12 hours. How many hours do they need working together?

Answer and explanation

Correct answer: A. 4

  1. Step 1: In one hour A does 1/6 of the job and B does 1/12.
  2. Step 2: Together they do 1/6 + 1/12 = 18/72 of the job per hour, so the job takes 72/18 = 4 hours.
Answer
4.
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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