Question 839194
Not said how the valves are used, maybe both are used simultaneously to drain the 6000 liters.  The rates are combined using addition.


{{{(1/1+1/(1/4))=5/4}}} liters per minute.
This is {{{(5/4)*60=75}}} liters per hour.


Let t = How many hours to drain the 6000 liters of water.
{{{highlight(75*t=6000)}}}
Solve for t