Pipe A can fill a tank in 60 minutes and Pipe B can empty the tank in 120 minutes.How long will they take to fill the tank if both pipes are opened simultaneously?

Pipe A can fill in 1 hour(60 minutes) is 1/1 or full of the tank.
Pipe B can empty in 1 hour (1/2) of the tank [120 mins= 2hrs]
Both pipes together can fill the tank in 1 hour = 1/1 - 1/2
= 1/2 of the tank.
Since 1/2 part of the tank is filled in 1 hour, the remaining part left is 1/2 of the tank.
The remaining 1/2 part will be filled in another 1 hour.
So both the pipes take 2 hours (120 minutes) to fill the tank.