Answer:
3 sides 180 degrees total
4 sides 180(4-2)
5 sides 180(5-2)
Step-by-step explanation:
n sides angle sum is 180(n-2)
21 sides angle sum is 180(21-2) = 180*19 = 3420
each angle in a regular 21-gon will be 3420/21 = 162.8571429 degrees (apprrox)
external angle = 360 - 162.8571429 = 197.1428571degrees = 197 degrees 8 minutes and 34.29 seconds approximately