Multiple choice general knowledge math & puzzles

A tank has four pipes attached to it. The first one takes 60 hours to fill the tank, second one takes 80 hours to do the same, third one takes 40 hours to empty the tank and the fourth one takes 720 hours to empty the tank. Four taps are opened at the same time. How long will it take to fill the tank if the tank is half-filled before opening the taps?

  1. 7 Days and 12 Hours

  2. 7 Days and 10 Hours

  3. 7 Days and 11 Hours

  4. 7 Days and 13 Hours
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Combined rate: 1/60 + 1/80 - 1/40 - 1/720 = (12+9-18-1)/720 = 2/720 = 1/360 per hour. Half tank needs 0.5 tank at 1/360 rate = 180 hours = 7 days 12 hours. The fill pipes (1,2) together are faster than drain pipes (3,4), so net effect is filling.

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