Hose B pours water in the pool for 3 minutes alone, at the rate of 250 liters
per minutes.  Therefore, hose B pours in (3)(250)=750 liters before A is turned
on.

Then there are 4350-750=3600 more liters to fill in.

Now hose A and hose B's combined filling rate is the sum of their individual
filling rates, or 150+250=400 liters per minute.

So  

to finish filling the pool.

So the total time is 3+9=12 minutes.

Edwin