<PRE><B>Question 182103</B>
A water tank has an inlet pipe and a drain pipe.
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A full tank can be emptied in 30 minutes if the drain
is opened and an empty tank can be filled in 45 minutes
with the inlet pipe opened. 
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If both pipes are accidentally opened when the tank is full, 
then how long will it take to empty the tank?
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Drain pipe DATA:
time = 30 min/job ; rate = (1/30) job/min
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Fill pipe DATA:
time = 45 min/job ; rate = 1/45 job/min
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Together DATA:
time = x min/job ; rate = 1/x job/min
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Equations:
fill rate - drain rate = together emptying rate
1/30 - 1/45 = 1/x

45x-30x 30*45
15x = 30*45
x = 90 min (time to empty the tank when both pipes are open
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Chers,
Stan H.

</PRE>