Answer:
As the height of the stairs starting from the bottom gets progressively taller by increments of 7, the height will vary. So we've got to be cautious when finding the volume for each and every individual step, especially the height.
Finding volume for the first step on the stairs;
Height: 7
Width: 40
Length: 10
Multiply:- (7 x 40 x 10) = 2,800
Finding volume for the second step on the stairs;
Height: 14 (the 7 doubled because it got taller)
Width: 40
Length: 10
Multiply:- (14 x 40 x 10) = 5,600
Finding volume for the third step on the stairs;
Height: 21 (the height of 7 tripled)
Width: 40
Length: 10
Multiply:- (21 x 40 x 10) = 8,400
Now we add all the volumes we've gathered so far;
2,800 + 5,600 + 8,400
= 16,800 cubic inches