.
You can use the formula from Physics
H = ,
where g = 32 ft/s^2 is the gravity acceleration near the Earth surface.
Alternatively, you may sum first 20 terms of the arithmetic progression with the first term of 16 feet and the common difference of 32 feet.
You will get the same result.
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For the issue on how these two approaches are connected, see the lessons
- Free fall and arithmetic progressions
- Uniformly accelerated motions and arithmetic progressions
- Increments of a quadratic function form an arithmetic progression
in this site.