Hey there,
Decagon - 
(10 - 2) x 180 degrees
 = 8 x 180 degrees
 = 1,440 degrees (sum of angles)
1 angle = 1,440 degrees / 10
 = 144 degrees
Pentagon - 
(5 - 2) 180 degrees = 3 x 180 degrees
 = 540 degrees (sum of angles)
1 angle = 540 degrees / 5 
 = 108 degrees
Do decagon - 
(12 - 2 ) x 180 degrees = 10 x 180 degrees
 = 1,800 degrees (sum of angles)
1 angle = 1,800 / 12
 = 150 degrees
16 - gon -
( 16 - 2) x 180 degrees = 14 x 180 degrees
 = 2,520 degrees (Sum of angles)
1 angle = 2,520 degrees / 16
 = 157.5 degrees
25 - gon -
(25 - 2) x 180 degrees = 23 x 180 degrees
 = 4,140 degrees (sum of angles)
1 angle = 4,140 / 25
 = 165.6 degrees
The formula = (n - 2) x 180 degrees
Hope this helps :))
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