The angles of the inside of a triangle always add up to 180 degrees (angle a + angle b + angle c = 180). So with that being said...
#19) 180 - (60 + 40) = 80
#20) 180 - (42 + 89) = 49
#21) 180 - (60 + 35) = 85
#22) sorry i can't see the numbers
#23) 180 - (48 + 67) = 65
#24) 180 - (35 + 90) = 55
^
(this is 90 because a square in the place of an angle signifies a right angle, or 90 degrees)
