Question 329130
Let the volume of the tank be V litres

Now, A full tank can be emptied in 30 minutes if the drain is opened

so in one minute V/30 litres will be drained.

and an empty tank can be filled in 45 minutes with the inlet pipe opened

so in one minute V/45 litres will be filled into the tank. 

now both pipes are opened when the tank is full.

when both pipes are opened per minute V/45 litres enter and V/30 litres exit the tank.

so in net V/30 - V/45 litres exit per minute

that is V/90 litres  per minute are drained from the tank when both pipes are opened .

so for the whole tank that is V litres to be emptied,
the time required will be

                 = V/(V/90)
                 = 90 mins

so when the both pipes are opened the time taken to empty the full tank is 90 minutes.

Answer: 90 minutes