Question

The three angles of a quadrilateral are 80°, 70° and 120°. The fourth angle is -

• A. ${110}^{\circ }$
• B. ${100}^{\circ }$
• C. ${90}^{\circ }$
• D. ${80}^{\circ }$

Answer 1

The correct option is C

${90}^{\circ }$

Let the fourth angle be $x^{\circ }$.

We know that the sum of all the angles of a quadrilateral is 360°.

${80}^{\circ }+{70}^{\circ }+{120}^{\circ }+x=360$

$\Rightarrow {270}^{\circ }+x={360}^{\circ }$

$\Rightarrow x={360}^{\circ }–{270}^{\circ }$

$\Rightarrow x={90}^{\circ }$

Thus, the fourth angle is ${90}^{\circ }$.