Two pipes can fill a tank in 10 minutes and 30 minutes respectively

Two pipes can fill a tank in 10 minutes and 30 minutes respectively and a third pipe can empty the full tank in 20 minutes. If all the three pipes are opened simultaneously, the tank will be filled in

On the basis of above given question ,

Part of the tank filled in 1 minute by pipe 1 =	1
	10

Part of the tank filled in 1 minute by pipe 2 =	1
	30

Part of the tank filled in 1 minute by pipe 3 = -	1
	20

When all the three pipes are opened simultaneously,

Part of the tank filled in 1 minute =	1	+	1	-	1	=	6 + 2 - 3	=	5	=	1
	10		30		20		60		60		12

Hence, the tank will be filled in 12 minutes.