Let x be the smallest angle
Then largest angle: 12x
3rd angle: 12x-20
You know all the angles of a triangle add up to 180°.
x+12x+12x-20=180
25x=180+20
25x/25=200/25
x=8
Smallest angle: 8°
Largest angle: 12(8) —> 96°
3rd angle: 12(8)-20 —> 76°
Hope I helped :)