The three angles of a quadrilateral are 80°, 70° and 120°. The fourth angle is
(a) 110°
(b) 100°
(c) 90°
(d) 80°

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Solution

(c) 90°

The three angles of a quadrilateral are 80°, 70° and 120°.
Let the fourth angle be x.
We know that the sum of all the angles of a quadrilateral is 360°.
80° + 70° + 120° + x = 360°
⇒ 270° + x = 360°
⇒ x = 360° − 270°
⇒ x = 90°
Thus, the fourth angle is 90°.

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