Question 981395
Every minute, one tap fills (1/15) of the tank and the other empties (1/20)
That is (1/15)-(1/20)=(1/60), by using 60 as a common denominator.

Every minute (1/60) th of the tank is filled.
That would require 60 minutes to fill the whole tank or 30 minutes to fill half the tank.

b.  Thirty minutes