Question

20. Pump A can fill a tank of water in 4 hours. Pump B can fill the same tank in 6 hours. Both pumps are started at 8:00 a.m. to fill the same empty tank. An hour later, pump B breaks down and took one hour to repair and was restarted again. When will the tank be full? (round your answer to the nearest minute).

Answer 1

Pump A fills 1/4 of the tank per hour, and pump B fills 1/6 of the tank per hour. Working together, then, they fill 5/12 of the tank in an hour. As a result, after one hour, they fill 5/12 of the tank. After the next hour, 2/3 is filled, because only 1/4 of the tank is filled since Pump B is broken. In the third hour, 1/3 of the tank is left. This takes (1/3)/(5/12), or 4/5, of an hour. This is 48 minutes. Therefore, it takes two hours and 48 minutes to fill the tank, finishing at 10:48.