To determine the volume of the tank, we need to know the volume of the marbles and the amount of water that Andrei wants to add to the tank. Based on the information given, we know that the marbles have a volume of 0.04 liters each, and that Andrei wants to fill the tank with 33 liters of water.
To calculate the volume of the tank, we need to add the volume of the marbles and the volume of the water that Andrei wants to add. If Andrei uses 50 marbles, the total volume of the marbles will be 50 x 0.04 = 2 liters. When we add the 33 liters of water that Andrei wants to add to the tank, we get a total volume of 2 + 33 = 35 liters. Therefore, the volume of the tank is 35 liters.
If Andrei has exactly 20 liters of water, he will need to add more marbles to the tank in order to fill it completely. To calculate the number of marbles that Andrei needs, we need to subtract the volume of the water from the total volume of the tank. Since the volume of the tank is 35 liters, and Andrei has 20 liters of water, he will need to add 35 - 20 = 15 liters of volume to the tank. Since each marble has a volume of 0.04 liters, Andrei will need 15 / 0.04 = 375 marbles to fill the tank completely. Therefore, Andrei will need 375 marbles if he wants to fill the tank completely with 20 liters of water.