Who can answer this IQ ? just try...

There is a water tank.
This water tank has two pipelines.
One is A and the other one is B.
A is to fill water and B is to remove water in the tank.
I turned off B and turned on A.
Then A pipeline took 3 hours to fill water to the entire tank.
Next I turned off A and turned on B.
B pipeline took 12 hours to remove entire water in the tank.

So if I open these two pipelines at the same time, how long will it take to fill water to the entire tank ?