An electric pump can fill a tank in 3 hours. Because of a leak in the tank, it took 3 hours 30 min to fill the tank. In what time the leak can drain out all the water of the tank and will make tank empty ?  Choices:
A) 10 hours
B) 13 hours
C) 17 hours
D) 21 hours
E) 23 hours
Explanation:   We can get the answer by subtrating work done by leak in one hour by subtraction of filling for 1 hour without leak and with leak, as   Work done for 1 hour without leak = 1/3   Work done with leak =   3 1/2=7/2   Work done with leak in 1 hr= 2/7   Work done by leak in 1 hr =1/3−2/7=1/21   So tank will be empty by the leak in 21 hours.   Option 21 hours
<result>D</result>
An electric pump can fill a tank in 3 hours. Because of a leak in the tank, it took 3 hours 30 min to fill the tank. In what time the leak can drain out all the water of the tank and will make tank empty ?