Two pipes A and B can fill a tank in 6 hours and 4 hours respectively. If they are opened on alternate hours and if pipe A is opened first, in how many hours, the tank shall be full?
Answer: B
LCM of 6 and 4 is 12. Total work = 12 units. A's rate=2, B's rate=3 units/hour. In the first hour, A fills 2 units. In the second hour, B fills 3 units. In a 2-hour cycle, 5 units are filled. After 2 cycles (4 hours), 10 units are filled. Remaining work is 2 units. It is A's turn. A will fill the remaining 2 units in 1 hour. Total time = 4 + 1 = 5 hours.