JAMB 1987 · UME · Q33

Three angles of a nonagon are equal and the sum of the six other angles is $1110^\circ$ . Calculate the size of one of the equal angles.

Worked solution (try it first)

The interior angles of a nonagon (9 sides) add up to $(9 - 2) \times 180^\circ = 1260^\circ$ .
The three equal angles share what is left: $1260^\circ - 1110^\circ = 150^\circ$ .
So each one is $150^\circ \div 3 = 50^\circ$ , option D.