Three angles of a quadrilateral are equal to $90^{\circ}$. The fourth angle is:

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

Solution

$360 - 3(90) = 360 - 270 = 90^{\circ}$. The quadrilateral is in fact a rectangle.

Asked in: IMO

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