Brookner has 3 pipes. One can fill a pool in 3 hours. Another can drain the same pool in 4 hours. The last one can fill the same pool in 9 hours. If all three pipes are left open, how long will it take, in hours, to fill an empty pool?

Question

Brookner has 3 pipes. One can fill a pool in 3 hours. Another can drain the same pool in 4 hours. The last one can fill the same pool in 9 hours. If all three pipes are left open, how long will it take, in hours, to fill an empty pool?

whpalmer4 · Answer

First pipe has a rate of 1 pool / 3 hours
Second pipe has a rate of -1 pool / 4 hours
Third pipe has a rate of 1 pool / 9 hours

Add up the rates, find the time to fill 1 pool.

anonymous · Answer

$$1/3 -1/4 +1/9$$=7/36

 7x/36 = 1
x = 36/7

is that correct?

anonymous · Answer

@whpalmer4

whpalmer4 · Answer

That is correct.