Question 1124472
Let me suppose the tank's volume is V litres. Then
speed of x: +V/10 litre/min 
speed of y: -V/6 litre/min
/*The positive/negative sign shows the increase/decrease of water volume.*/
Initially the water is 3V/4. Let's begin.
3V/4+V*t/10-V*t/6=0
t=45/4=11.25 minutes.

And we're done. Figure out what we want. Extract useful material from the question. Design the approach. Do.