Question 951005
PIPE_______________RATES
Inlet_______________{{{1/10}}}
Outlet_____________{{{-1/30}}}
Combined__________{{{1/10-1/30=1/15}}}


"If the pool is 15 filled and then the..."


Fifteen what?  This description need to be more specific or nothing can be solved.
Is this the numerator of a fraction?  Fraction of what?  Is this percent of the pool?
Is it a specific volume  (not likely)?  Is this a time quantity?  If time, then is
this minutes or hours?


Here is a guess the piece of information is "15 minutes".


If fill pipe was open for 15 minutes, this is {{{1/4}}} hour, and you can ask what is t in hour
to fill the pool after both pipes became open.
-
{{{(1/10)*(1/4)}}}, amount of pool filled already
{{{highlight_green((1/15)*t=1-(1/10)(1/4))}}}, amount still to be filled
;
Solve for t.