The sum of the three angles of a triangle is always 180°. Tha is
(first measure) + (second measure) + (third measure) = 180°
Subtract the first two terms of this equation and you have
(third measure) = 180° - (first measure) - (second measure)
a. third measure = 180° - 45° - 55° = 80°
b. third measure = 180° - 50° - 40° = 90°
c. third measure = 180° - 110° - 35° = 35°
