Question

if a plane mirror is fixed at and the incident ray is turned by an angle 30° in clockwise direction then​

Answer 1

It means that angle of reflection is 30 degree too.

As in plane mirror,

angle of incidence=angle of reflection

,i=r

Answer 2

Draw a diagram of reflection by a plane mirror.

see, that if mirror is rotated through an angle theta , the reflected ray turns through an angle two times theta.

this is due to the incidence and reflected both angles turning through theta. as the normal to the mirror also rotates.

in the present case

you are rotating the mirror through an angle 30 degrees, so the reflected ray will rotate in the above manner by an angle 60 degrees.

however you have also rotated incidence ray by 10 degree. if it is in the same direction as the turning of mirror , this will reduce the angle of incidence by 10 degree , so the actual rotation of the reflected ray will be reduced by 10 degree,

so finally you will get the reflected ray turned through

60 -10 = 50 degrees.