Question

Two pipes M & N can fill a cistern in 12 & 16 hrs respectively If both the pipes are opened together then after how many minutes N should be closed so that the tank is full in 9 hours

Answer 1

Let N be closed after x hrs Then $x(\frac{1}{12}+\frac{1}{16})+(9-x)\frac{1}{12}=1$
$\Rightarrow x=16\times \frac{3}{12}\Rightarrow$ x = 4 hrs = 240 minutes