Question 935598
You mean, 20 hours with the slow tank and then 20 hours with both tanks filled the pool to {{{2/3}}} full?  Otherwise, too many unknowns.



Rates, POOLS per HOUR
Slow pipe, {{{1/x}}}
Fast pipe, {{{1/(x-24)}}}
Basic rule for filling a tank, RT=V rate time the volume capacity.



{{{highlight_green((1/x)*8+(1/x+1/(x-24))*20=2/3)}}}.
If you can manage the arithmetic then solving for x is what you want.
Most important is to understand the equation first.