Question 79963: 49. One pipe can fill a tank in 8 minutes and a second pipe in 12 minutes. If the tank is empty, how long will it take both pipes to fill it?
Answer by doctor_who(15)   (Show Source):
You can put this solution on YOUR website! We don't know the volume of the tank, so let's say it has a capacity of G gallons.
If the first pipe can fill the tank in 8 minutes, then the flow rate of the pipe is G/8 gallons per minute.
Likewise, the flow rate of the second pipe is G/12 gallons per minute.
That means the combined flow rate is (G/8) + (G/12) gallons per minute.
Now, put them over a common denominator of 24 and simplify :
(G/8) + (G/12) = 3G/24 + 2G/24
= (5/24)G.
So every minute, 5/24 of the tank gets filled by both pipes together.
Therefore, in 24/5 minutes, the whole tank will be filled.
24/5 minutes = 4 minutes and 48 seconds (ANSWER)
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