Two pipes P and Q can fill a cistern in 12 and 15 minutes respectively. Both are opened together, but at the end of 6 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 one.
A) 1
B) 1.5
C) 2
D) 2.5
E) 3

Rationale: "let x be the total time it takes for the cistern to fill . 6 / 12 + x / 15 = 1 x / 15 = 1 / 2 x = 7.5 after the first pipe is turned off , it takes 1.5 more minutes to fill the cistern . the answer is b ."
Answer: B