Volume of water that enters the tank is 20m^3/h. Level of water inside the tank is 50% wich is equal to 400m^3. Pump is able to deliver 30m^3/h . how long will it take to empty the tank ?

Answer 1

The net rate of draining is 30-20 = 10 m^3/hr

assuming that there are in fact 400m^3 of water in the tank, it will drain in
400/10 = 40 hr

although, it is unclear whether there is only 50% of 300 m^3 in the tank. Adjust the result as needed.