Answer:
The total amount of water needed is 540 cubic inches.
Step-by-step explanation:
A cubic prism has three dimension. Lenght l, width w and height h.
It's volume is:

In this question:

Dimensions in inches, so the volume will be in cubic inches.

The volume is 600 cubic inches.
What is the total amount of water needed, to the nearest cubic inch, to fill 90% of the aquarium with water?
This is 90% of 600. So
0.9*600 = 540
The total amount of water needed is 540 cubic inches.