The time it will take for the vat to get full if both pipes are open is;
t = ²⁰/₃ hours
We are told that;
Inlet pipe can fill the vat in 4 hours
Outlet pipe can empty in 10 hours
1/4 - 1/10 = 1/t
I used negative in front of 1/10 because the outlet is taking water out of the tank
Thus;
1/4 - 1/10 = 1/t
Multiply through by 40 to get;
10 - 4 = 40/t
6 = 40/t
Cross multiply to get;
t = 40/6
t = ²⁰/₃ hours
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