Ok so for the first one: -We know that the 4 angles add up to 360 -3 of the angles are given -To find the fourth we simply subtract the sum of the 3 angles from 360.
Equation: 120+ 100+ 100 + x = 360 = 320+x= 360
Now by making x the subject x= 360 - 320 = 40 degrees
