Question 1177723
Let pump B to empty a pool be x hours
It empties 1/x in 1 hour

pump A to empty a pool will be x-2  hours
It empties 1/(x-2) in 1 hour
Pump B runs for two hours and both work for two hours
They empty 40 % of the tank
{{{2/x + 2/(x-2) =2/5}}}

{{{(2x-4 +2x)/x(x-2) = 2/5}}}

{{{5(4x-4)=2(x^2-2x)}}}

{{{20x -20 = 2x^2-4x}}}

{{{2x^2-24x+20=0}}}
factorise

Pump B takes 11 hours 
Pump A takes 9 hours

Pump A has to empty remaining water (3/5) of the tank

Pump A will take 3/5   * 9  = 27/5 = 5.4 hours after 12.00