Two pipes A and B can fill a water tank in 20 and 24 minutes

Two pipes A and B can fill a water tank in 20 and 24 minutes respectively and a third pipe C can empty at the rate of 3 gallons per minute. If A, B and C are opened together to fill the tank in 15 minutes, the capacity (in gallons) of the tank is :

Let the capacity of the tank be q gallons.
When all the pipes A, B and C are opened simultaneously

Quantity of water filled in the tank in 1 minute =	q	+	q	- 3
	20		24

According to the question ,

	q	+	q	- 3 =	q
	20		24		15

⇒	q	+	q	-	q	= 3
	20		24		15

⇒	6q + 5q - 8q	= 3
	120

⇒ 3 q = 3 × 120

⇒ q =	3 × 120	= 120 gallons
	3