The sum of the angles = 360.
Add the 4 angles together so they equal 360:
1/3x + x-20 + x-10 + 40 = 360
Combine like terms:
2 1/3x - 30 + 40 = 360
Continue combining like terms:
2 1/3x +10 = 360
Subtract 10 from both sides:
2 1/3x = 350
Divide both sides by 2 1/3:
x = 150
