Question

Two pipes p and q can fill a tank in 12 minutes and 16 minutes respectively. if both the pipes are opened simultaneously, after how many minutes should q be closed so that the tank is full in 9 minutes

Answer 1

Pipe P and Q will be open for initial xxminutes then P will be open for another (9-x) min
x(112+116)+(9−x)112=17x48+912−x12=17x+36−4x=483x=12x=4