The total amount of water that he can fill is the volume of the aquarium (70 × 50 × 30 = 105 000cm³)
Now, since he fills it three-quarters, we need to multiply the total volume by 3/4.
Hence, 105 000 × 3/4 = 78750cm³
To find the total amount of litres, we simply divide by 1000 = 78.75 L