Two pipes can independently fill a bucket in 20 minutes and 25 minutes. Both are turned on together for 5 minutes after which the second pipe is turned off. What is the time taken by the first pipe alone to fill the remaining portion of the bucket ?

Let capacity of bucket = L.C.M. (20,25) = 100 litres

First pipe can fill it in 20 minutes, => first pipe's efficiency = $$\frac{100}{20}=5$$ l/min

Similarly, second pipe's efficiency = $$\frac{100}{25}=4$$ l/min

=> Volume of bucket filled by both in five minutes = $$(5+4)\times5=45$$ litres

$$\therefore$$ Time taken by the first pipe alone to fill the remaining portion of the bucket = $$\frac{(100-45)}{5}=11$$ minutes

=> Ans - (A)