Two pipes can fill a tank in 20 and 24 minutes respectively. There is an outlet in the tank that can empty it at 3
gallons per minute. If all the pipes are opened, the tank gets filled in 15 minutes. Find the capacity of the tank.

Let say, the capacity of tank is = x ltr.

First pipe :

it can fill in 15 min = $$\frac{15x}{20}\ ltr.$$

Second pipe :

it can fill in 15 min = $$\frac{15x}{24}\ ltr.$$

Last pipe :

it can empty in 15 min = $$3\times15\ ltr=\ 45\ ltr.$$

So, 

$$\frac{15x}{20}+\frac{15x}{24}-45=x\ .$$

or,$$\frac{15x}{20}+\frac{15x}{24}-x=45.$$

or,$$\frac{90x+75x-120x}{120}=45.$$

or,$$\frac{45x}{120}=45.$$

or,$$x=120.$$

A is correct choice.