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.

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- Dec 21st 2012, 08:04 AMhisajeshPipes and cistern..need your answer.
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 AMskeeterRe: Pipes and cistern..need your answer.
- Dec 21st 2012, 08:58 AMhisajeshRe: Pipes and cistern..need your answer.
Sorry typo...it is 4 hours longer*

- Dec 21st 2012, 09:29 AMHallsofIvyRe: Pipes and cistern..need your answer.
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 AMskeeterRe: Pipes and cistern..need your answer.
rate of A = (1 tank)/(12 hrs)

rate of B = (1 tank)/(t hrs)

8(rate of A + rate of B) = 1 tank filled

I agree with your solution ...

8[(1/12) + (1/24)] = 1