JAMB 2015 · UTME · Q15

A pentagon has four of its angles equal. If the size of the fifth angle is $60^\circ$ , find the size of each of the four equal angles.

Worked solution (try it first)

The interior angles of a pentagon add up to $(5 - 2) \times 180^\circ = 540^\circ$ .
The four equal angles share $540^\circ - 60^\circ = 480^\circ$ .
So each is $480^\circ \div 4 = 120^\circ$ , option C.