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? Choose the correct choice from the following choices
A) 3 hrs
B) 5 hrs
C) 2 hrs 15 min
D) 4hrs 24 min
E) 4 hrs
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= 
<gadget id="calculator">2/5 * 60</gadget>
<output>24</output>
24= 4 hrs 24 min   Answer: 4hrs 24 min
<result>D</result>
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?