Question 58150
He leaves at 12:40P.M.
Here you need to know to facts. One pipe can fill the whole tank by itself in 20 minutes, while the other requires 30 minutes to do the same task.
When Wilbur finishes his lunch, he returns to see how full the tank is now. It is nearly full, so he decides to wait until the water reaches its proper height in the tank. At the moment it does, he turns off the two pipes and notes the time on his watch.
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1st Pipe DATA:
Time to fill tank = 20 min/job ; rate = 1/20 job/min
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2nd Pipe DATA:
Time to fill tank = 30 min/job ; rate = 1/30 job/min
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Pipes together DATA:
Time to fill tank = x min/job ; rate = 1/x job/min
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EQUATION:
1st pipe rate + 2nd pipe rate = together rate
1/20 +1/30 = 1/x
50/600 = 1/x
600/50=x
x=12 minutes (The two pipes together will take 12 min to fill tank)
He should shut them off at 12:40 + 12 min = 12:52 PM
Cheers,
Stan H.