When you are given the side length and apothem of a regular polygon and asked for area, you are expected to use the formula
... A = (1/2)Pa
where P is the perimeter of the polygon, "a" is the apothem, and A is the area.
The only additional information you need is that a decagon has 10 sides, so its perimeter is 10×3.2 inches = 32 inches.
The area is ...
... A = (1/2)(32 in)(5 in)
... A = 80 in²
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Sometimes the dimensions of a problem like this don't quite add up. The geometry of a regular polygon means that the apothem can be found from the side length and vice versa. In this case, the side length of 3.2 inches means the apothem is about 4.92 inches; or the apothem of 5 inches means the side length is about 3.25 inches. Either way, the area is slightly different from that computed using the given numbers.
This is why I say you're expected to use the above formula. (You're not expected to think about it too much.)
