I'm not sure if the author can do basic math. It's true that all 4 sided shapes have angles that add up 360 degrees and that 120 and 60 adds up to 180, but the right angle labeled 100 degrees should say 90 because all right angles are 90 degrees. So the bottom angle not shown should be 90 degrees for 2 reasons, first the missing angle is also drawn as a right angle, second the other angles 120 and 60 add up to 180 and all angles must add up to 360 so, 360 - 180 - 90 = 90. it's not hard, it's elementary school math.
side note: if that mislabel right angle was really 100 then the missing angle would have been 80. 360 - 180 - 100 = 80