Two pipes A and B can fill a tank in 4 and 5 hours respectively. If they are turned up alternately for one hour each, the time taken to fill the tank is?

A. 2 hrs 15 min
B. 4 hrs 24 min
C. 5 hrs
D. 3 hrs
Explanation:
1/4 + 1/5 = 9/20
20/9 = 2 2/9
9/20 * 2 = 9/10 —- 4 hours
WR = 1 – 9/10 = 1/10
1 h —- 1/4
? —– 1/10
2/5 * 60 = 24 = 4 hrs 24 min