Its quite an unfair type of question for some people. E.g. Those who went to postgraduate studies etc, who might be out of touch with basic high school physics after so many years of not using it.
The trick is to realize that all the lengths are preserved in the mirror's reflection, and pretend the mirror is a hole into which you are looking at another object.
1) If we represent this the X,Y axis, with the line x = 0 representing the wall, and x = d representing the person. Then, the reflection of the person is the line x = -d.
2) Then, because he can just see his feet in the mirror: draw a line from the top of the person's head to the feet of his reflection. Imagine if another person is actually standing there and you can only see down to just his feet through the "hole".
3) This creates two congruent triangles. Giving the result t = 2h, regardless of d.