Question 646694
In a water refilling station, one pipe can fill a tank in 30 min. while another pipe can fill it in 40 min. A third pipe can empty the same tank in 20 min. If all pipes are opened at the same time. in how many minutes will the tan be filled?
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one pipe can fill a tank in 30 min.
Adds 1/30 of a tank per minute.
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while another pipe can fill it in 40 min.
Adds 1/40 of a tank per minute.
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A third pipe can empty the same tank in 20 min
Subtracts 1/20 of a tank per minute.
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Each minute, 1/30 + 1/40 - 1/20 of the tank
= 4/120 + 3/120 - 6/120 = 1/120
--> 120 minutes