JAMB 2002 · UME · Q18

The sum of the interior angles of a polygon is 20 right angles. How many sides does the polygon have?

Worked solution (try it first)

20 right angles is $20 \times 90^\circ = 1800^\circ$ .
The interior angles of an $n$ -sided polygon add up to $(n - 2) \times 180^\circ$ , so $(n - 2) \times 180 = 1800$ and $n - 2 = 10$ .
So $n = 12$ , option B.