Multiple choice

Two pipes, A and B can fill a tank in 15 hr and 20 hr, respectively. How long will it take to fill the tank if both the pipes are used together?

  1. 12 1 2 hr

  2. 17 1 2 hr

  3. 8 4 7 hr

  4. 15 4 7 hr
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Combined rate = 1/15 + 1/20 = 4/60 + 3/60 = 7/60. Time = 60/7 = 8 and 4/7 hours.

AI explanation

Using the method of finding the combined work rate, the first part of the tank filled in one hour by pipe A is 1/15 and by pipe B is 1/20. Working together for one hour, they fill 1/15 + 1/20, which equals 7/60 of the tank. The total time taken to fill the tank is the reciprocal of the one hour work, so 60 divided by 7 gives 8 4/7 hours.

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