Question 223158
 if nine large pipes can drain a pond in eight hours, and six small pipes can drain the same pond in sixteen hours, how long will it take three large pipes and five small pipes to drain the pond?
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I assume all "large pipes" are the same size, and same for small pipes.
9 large pipes take 8 hours --> 1/8 of the pond drained per hour for 9
--> 1/72 pond per hour per large pipe.
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6 small pipes take 16 hrs --> 1/16 pond/hr for 6
--> 1/96 pond/hr per small pipe.
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3 large can do 3/72 ponds/hr = 1/24
5 small can do 5/96 ponds/hr
1/24 + 5/96 = 9/96 ponds/hr total = 3/32 ponds/hr
--> 32/3 hours per pond
= 10 hours 40 minutes