Question 439419: A swimming pool holds 540,000 liters of water. THe pool has two drainage pipes. When the pool is completely full, the first pipe alone can empty it in 180 min, and the second pipe alone can empty it in 120 min. When both pipes are draining together, how long does it take them to empty the pool?
Found 3 solutions by mananth, ikleyn, josgarithmetic:
Answer by mananth(16949)   (Show Source):
You can put this solution on YOUR website! Pipe A 3 hours
It does 1/ 3 of the job in 1 hour
Pipe B 2 hours
It does 1/ 2 of thejob in 1 hour
Together they will do 1/ 3 + 1/ 2
Together they will do 5/ 6 of the job in one hour
So they will take 1 1/5 hours
So they will take 1.2 hours = 72 minutes
Answer by ikleyn(53751)  (Show Source):
You can put this solution on YOUR website! .
A swimming pool holds 540,000 liters of water. The pool has two drainage pipes.
When the pool is completely full, the first pipe alone can empty it in 180 min,
and the second pipe alone can empty it in 120 min.
When both pipes are draining together, how long does it take them to empty the pool?
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The technology, which @mananth uses in his solution, is based on using fractions,
so it is the Pre-Algebra level of about 6th or 7th grade.
Meanwhile, the problem itself is pure Arithmetical and can be solved by a 4-th grade student.
Moreover, this problem is a standard problem of this type and is specially designed for the 4th grade level
and is expected to be solved via Arithmetic.
So, I will give an Arithmetic solution, as it is designed/assumed by the problem.
First pipe drains  = 3000 liters of water per minute.
Second pipe drains  = 4500 liters ow water per minute.
Working together, the two pipes drain 3000 + 4500 = 7500 liters of water per minute.
So, the time for 2 pipes to drain the full amount of water from the pool is
 = 72 minutes.
ANSWER. 72 minutes for two pipes, or 1 hour and 12 minutes.
Solved at the level for which this problem is really designed.
Answer by josgarithmetic(39792)  (Show Source):
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