Pipe A usually fills a tank in 2 hours. On account of a leak at the bottom of the tank, it takes pipe A 30 more minutes to fill the tank. How long will the leak take to empty a full tank if pipe A is shut? Choose the correct answer:
A) 7 hours  B) 8 hours  C) 9 hours  D) 10 hours  E) 11 hours

Rationale: pipe a fills the tank normally in 2 hours . therefore , it will fill ½ of the tank in an hour . let the leak take x hours to empty a full tank when pipe a is shut . therefore , the leak will empty of the tank in an hour . the net amount of water that gets filled in the tank in an hour when pipe a is open and when there is a leak = of the tank . — ( 1 ) when there is a leak , the problem states that pipe a takes two and a half hours to fill the tank . i . e . hours . therefore , in an hour , of the tank gets filled . – ( 2 ) equating ( 1 ) and ( 2 ) , we get = > = > x = 10 hours . the problem can also be mentally done as follows . pipe a takes 2 hours to fill the tank . therefore , it fills half the tank in an hour or 50 % of the tank in an hour . when there is a leak it takes 2 hours 30 minutes for the tank to fill . i . e hours to fill the tank or or 40 % of the tank gets filled . on account of the leak , ( 50 - 40 ) % = 10 % of the water gets wasted every hour . therefore , the leak will take 10 hours to drain a full tank . answer : d
Answer: D