Hello, geeko!

Code:

40
- * - - - - - - - - - - - - - - - - - *
: | |
3 | | 3
: | 40 |
- * - - - - - - - - - - - - - - - - - *
: | x *
: + - - - - - - - - - - - *
6 |:::::::::::::::::*
: h|:::::::::::*
: |:::::*
- *

For the first six feet, the volume of water is a triangular prism.

. . Its volume is: .

From the similar right triangles: . .[1]

The area of the triangle is: .

Substitute [1]: .

Then the volume is: .

. .

For the last three feet of water, the bottom (triangular) portion is already filled.

. . It contains: . ft³ of water.

The upper portion is a rectangular slab of water:

. . length 40, width 30, and height

Its volume is: . ft³.

The total volume is: .

. .

Therefore: .