SOLUTION: A tap can fill a tank in 15 minutes,while another tap can empty the tank in 20 minutes.When the tank is already half full,both the taps are opened together.The tank will be filled

Question 981395: A tap can fill a tank in 15 minutes,while another tap can empty the tank in 20 minutes.When the tank is already half full,both the taps are opened together.The tank will be filled up in the next:
a)40 minutes
b)30 minutes
c)32 minutes
d)36 minutes.
Answer by Boreal(15235) (Show Source): You can put this solution on YOUR website!
Every minute, one tap fills (1/15) of the tank and the other empties (1/20)
That is (1/15)-(1/20)=(1/60), by using 60 as a common denominator.
Every minute (1/60) th of the tank is filled.
That would require 60 minutes to fill the whole tank or 30 minutes to fill half the tank.
b. Thirty minutes
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