One hose fills    of the tank volume per hour.


Another hose fills    of the tank volume per hour.


Working together, these hoses fill   =  =   of the tank volume per hour.


The draining pipe takes off  of the tank volume per hour.


You see that inflow rate of two hoses is greater than the outflow rate of the draining pipe.


Therefore, the net inflow rate is positive; so tank will be steadily filled.



    Notice that even each single hose brings more water that the draining pipe takes off,
    so there is no ANY doubts that the tank will be filled - it is out of the question.



Now, the precise net inflow rate is   =  =   of the tank volume per hour.


So, the tank will be filled in 4 hours, if both hoses and draining pipe are turned on.