This looks like a remarkably trivial Calculus problem- there no calculus involved at all.

Water will start to exit the tank when the height of the water in the tank equals the height of the hole, .8 m. As the water rises to, say, height h, the volume of water in the tank will be 1*1*h= h so we want to determine when the volume of water is .8 cubic meters. Since a liter the same volume as a cube 10 cm= .1 m on a side, a liter is cubic meters. Water is coming in at 20+2t liters= .001(20+ 2t) cubic meters per minute, water will start draining out the hole when .001(20+ 2t)= .8.

Solve that equation.