• Dec 21st 2012, 08:04 AM
hisajesh
Two pipes A and B can fill a tank in 8 hours. If only pipe A is open, then it would take 4 hours to fill the tank. Find how much longer would it take if only pipe B is open.

My answer is 24 hours but the book answer is 16, kindly help.
• Dec 21st 2012, 08:15 AM
skeeter
Quote:

Originally Posted by hisajesh
Two pipes A and B can fill a tank in 8 hours. If only pipe A is open, then it would take 4 hours to fill the tank. Find how much longer would it take if only pipe B is open.

My answer is 24 hours but the book answer is 16, kindly help.

something doesn't make sense here ... both pipes fill the tank in 8 hrs, but pipe A (by itself) fills the tank in 4 hrs ???
• Dec 21st 2012, 08:58 AM
hisajesh
Sorry typo...it is 4 hours longer*
• Dec 21st 2012, 09:29 AM
HallsofIvy
In other words pipes A and B together fill this in 8 hours and B alone fills it in 12 hours.

Whenever you have a problem like this, whether it is pipes or cars or whatever, it it the speeds that add. Let x be the time it would take for A alone to fill the cistern. The speed with which it fills is 1/x. Let y be the time it takes B alone to fill the cistern: y= 12 hours. The speed with which it fills is 1/y= 1/12 "cistern per hour". The two together fill the cistern in 8 hours so 1/x+ 1/12= 1/8.
• Dec 21st 2012, 09:34 AM
skeeter