I have a pyramid shaped dump with a square base of 100 yards. One yard vertically above the base, the length of the side parallel to the base is 99 yards; the dump can be built up to a height of 20 yards (the top of the pyramid is never reached).
I have a pyramid shaped dump with a square base of 100 yards. One yard vertically above the base, the length of the side parallel to the base is 99 yards; the dump can be built up to a height of 20 yards (the top of the pyramid is never reached).
The information given says that the "length of the side parallel to the base" decreases by one yard for every yard in height. Since it was 100 yards in length at the base, it will have decreased to 0 when the pyramid is 100 yards high.
Now, since you were told that "the dump can be built up to a height of 20 yards" and the actual height of the pyramid is irrelevant, what was the real problem?