The volume of one marble is 0.04 liters.
Multiply the volume by the number of marbles:
50 x 0.04 = 2 liters.
Add that to the volume of water:
33 +2 = 35 liters
The volume of the tank is 35 liters.
Part 2:
Subtract the volume of water from the volume of the tank:
35 - 20 = 15 liters.
Now divide the remaining volume by the volume of a marble:
15 / 0.04 = 375 marbles are needed.