Two pipes A and B can fill a tank in 20 hours and 24 hours respectively. If the two pipes opened at 5 in the morning, then at what time the pipe A should be closed to completely fill the tank exactly at 5 in the evening?

Let volume of tank = L.C.M. (20,24) = 120 units

Pipe A can fill the tank in 20 hours, => A's efficiency = $$\frac{120}{20}=6$$ units/hr

Similarly, B's efficiency = $$\frac{120}{24}=5$$ units/hr

Let pipe A is opened for $$t$$ hours, then in 12 hours 120 units of tank is filled.

=> $$(5\times12)+(6\times t)=120$$

=> $$6t=120-60$$

=> $$t=\frac{60}{6}=10$$

$$\therefore$$ Pipe A is closed after 10 hours, i.e. at 3 pm

=> Ans - (A)