Question 202142
Let x=amount of time it takes to fill the tank when both pipes are working
Then the tank fills at the rate of 1/x tank per min with both pipes working

The tank fills at the rate of 1/15 tank per min with the inlet pipe
The tank empties at the rate of 1/75 tank per min with the outlet pipe
So, our equation to solve is:
1/15 - 1/75=1/x or
5/75 - 1/75=1/x
4/75=1/x
4x=75
x=18.75 min

CK
(4/75)*18.75=75/75=1 (1 tank, that is)

Hope this helps---ptaylor