Two pipes A and B can fill a tank in 24 minutes, and 32 minutes. If both the pipes are opened simultaneously, after _______ minutes time B should be closed so that the tank is full in 18 minutes .

(Let B be closed after x minutes. Then,
Part filled by (A+B) in x min+ part filled by A in (18-x)min=1
x(1/24+1/32)+(18-x)*1/24=1
7x/96+18-x/24=1
7x+4(18-x)=96
x=8. Hence, B must be closed after 8 mins. )