Sainik School Entrance Exam 2021 Solved Paper

(c) In a triangle exterior angle is equal to the sum of interior opposite angles.

Interior opposite angles are in the ratio $1:3$.
Let the angles are $\mathit{x}$ and $3\mathit{x}$.
Then, $\mathit{x}+3\mathit{x}=60°$
$\Rightarrow 4\mathit{x}=60°\Rightarrow \mathit{x}=15°$
So, opposite interior angles are $15°,45°$ and third angle is $120°$.
(∵ sum of angles of a △ is $.180°)$.