Multiple choice general knowledge math & puzzles

Tap A and Tap B can fill a tank in 10 hours and 20 hours respectively. Tap C can empty the tank in 30 hours. Tap A and Tap B are opened for 5 hours. After that Tap C is opened. Find the total time taken in filling the tank upto 87.5% of its volume.

  1. 50/7 hours

  2. 85/14 hours

  3. 100/7 hours

  4. 95/14 hours
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Tap A fills 1/10 per hour, Tap B fills 1/20 per hour. In first 5 hours, A+B fill 5*(1/10+1/20) = 3/4 of tank. Remaining 13.5% is 27/200. With A+B+C running, net rate is 5/12 per hour. Time = (27/200)/(5/12) = 81/500 hours. Total = 5 + 81/500 = 85/14 hours.

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