To solve this problem, we must find the volume of the sandbox.

Where:

Now, we replace and solve:

The A answer is 3120 cubic meters
Now, for question "b" we must find how many cubic meters are 89% of its capacity, that is 89% of the capacity of the 3120 cubic meters.
For this, we multiply the total capacity of the sandbox by 0.89.

The A answer is 2776.8 cubic meters