A ray of light is incident on a horizontal plane mirror at an angle of incidence ${30}^o$. By what angle should the mirror be rotated so that the reflected ray becomes horizontal?

According to question, $i={30}^0$

And from laws of reflection $r=i={30}^0$

And from figure $\theta ={90}^0-r={90}^0-{30}^0$

$\theta ={60}^0$

And we know that if we rotate mirror by $\alpha$ then reflected ray will rotate by $2\alpha$. Hence if we rotate mirror by ${30}^0$ then reflected ray will rotates by ${60}^0$ to be horizontal.