The 3 inside angles of a triangle equal 180.
Add the 3 angles together to equal 180:
X + (5x-20) + 4x = 180
Simplify the left side by adding like terms:
10x-20 = 180
Add 20 to each side:
10x = 200
Divide both sides by 10:
x = 20
Angle A = x = 20
Angle B = 4X = 4(20) = 80
Angle C = 5x-20 = 5(20) -20 = 100-20 = 80
