Question 900943
Recognize, this is a uniform rates situation following RT=J for rate, time, Number_of_Tanks_to_Fill.


The fill pipe rate is 1/x.
The drain pipe FILL rate is -1/y.
Combined pipes fill rate is 1/3.
Also, combined pipes fill rate is {{{1/x-1/y=1/3}}}.


Run the pipes both for 1 hour and then just the fill pipe for {{{2/3}}} hour.  That fills the tank,  1 tank.


{{{highlight((1/3)*1+(1/x)(2/3)=1)}}}-----Note very carefully, x is the time for the "pipe" to fill the tank if the drain is NOT in operation.
Solve for x.  Unit is hours.