Question 862928
The filling is an opposite task than the emptying.  Work with the rates as TANK per HOUR.  


Pump A without leak, {{{1/3}}} tank per hour
Leak, {{{-1/x}}} tank per hour
Combined A and leakage, {{{1/3.5}}} tank per hour


Notice very carefully:
Filling the tank is positive, and emptying the tank is negative.
x is the time for the leak alone to empty the tank if started from full.
3.5 hours is the same as 3 hours 30 minutes.


The combined rate of pump A and the leak working at the same time is the sum of their individual rates:
{{{(1/3)+(-1/x)=1/3.5}}}
{{{1/3-1/x=1/3.5}}}
and you could prefer
{{{highlight(1/3+1/x=5/13)}}}-----SOLVE FOR x.