Question 909693
Let t be the unknown minutes during both pipes open together;


RATES------------------
Pipe A, 25 liters/minute
Pipe B, 40 liters/minute
A&B, 65 liters/minute


The question is a uniform rates situation, {{{highlight_green(RT=V)}}}, rate time volume.



Four minutes with just pipe A and then unknown t minutes for pipes A and B together.


{{{highlight(highlight(25*4+65*t=100))}}}

Solve for t.
The outlined equation is the most important part of the solution.