Three angles of a quadrilateral are equal and the fourth angle is $120^{\circ}$. Each of the equal angles is:

Three angles of a quadrilateral are equal and the fourth angle is $120^{\circ}$. Each of the equal angles is:
  1. $60^{\circ}$
  2. $70^{\circ}$
  3. $80^{\circ}$
  4. $90^{\circ}$

Solution

Let each equal angle be $x$. Then $3x + 120 = 360 \Rightarrow 3x = 240 \Rightarrow x = 80^{\circ}$.

Asked in: IMO

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