An electric pump can fill a tank in 3 hours. Because of a leak in ,the tank it took 3(1/2) hours to fill the tank. If the tank is full, how much time will the leak take to empty it ?
A. 20 hrs
B. 23 hrs
C. 30 hrs
D. 21 hrs
Correct Answer :
D. 21 hrs
work done by the leak in 1 hour=(1/3)-(1/(7/2))=(1/3)-(2/7)=(1/21).