Answer: 8 ft
<u>Step-by-step explanation:</u>

Step-by-step explanation:
Size of Interior angle = 180° - 360°/n, where n is the number of sides of the regular polygon.
We have 180° - 360°/n = 160°.
=> 360°/n = 20°, n = 18.
Therefore it is a 18-sided regular polygon.
I believe the answer is 6120.