Answer:
21.35

Explanation:
It is given that the box is a triangular prism.
Side of Base of prism = 7 ft
Height of Base of prism = 6.1 ft
Area of a triangle is given as:


Volume of a prism is:

B is the area of base of prism
h is the height of prism
Here, h = 1 foot
So, V = 21.35
1
Volume of prism is = 21.35

Sand required to fill the triangular prism shape box is 21.35
.