Two pipes P and Q can fill a cistern in 10 and 15 minutes respectively. Both are opened together, but at the end of 4 minutes the first is turned off. How many more minutes will it take for the cistern to fill after the first pipe is turned off?  Choose the correct answer.
A) 3.5	B) 4	C) 4.5	D) 5	E) 5.5
Let x be the total time it takes for the cistern to fill.   4/10 + x/15 = 1   x/15 = 3/5   x = 9   After the first pipe is turned off, it takes 5 more minutes to fill the cistern.   The answer is 5.
<result>D</result>
Two pipes P and Q can fill a cistern in 10 and 15 minutes respectively. Both are opened together, but at the end of 4 minutes the first is turned off. How many more minutes will it take for the cistern to fill after the first pipe is turned off?