Two pipes P and Q can fill a cistern in 12 minutes and 16 minutes respectively. Simultaneously both the pipes are opened together, then after how much time Q should be closed so that the tank is full in 9 min?

A. 3 min
B. 5 min
C. 2 min
D. 4 min

Then part filled by (P+Q) in x min + part filled by P in ( 9 – x )min
x(1/12+1/16)+(9−x)1/12=1