A spot light on ground shines on a wall 12 m away.

Height of the man is 2 m.

Man is moving towards the wall.

Speed of the man is  m/s.

Draw a triangle for the given situation:

Assume that man be at  m apart from the wall at any time .

Let the length of the shadow on the wall is .

Speed of the man is  m/s.

Here negative sign indicates that the distance is decreasing.

From the property of similar triangles,

.

.

Differentiate on each side with respect to .

.

Find how fast length of his shadow decreasing , when he is  m from the building.

Therefore,  and .

Substitute above values in .

Therefore, length of the shadow is decreasing at 0.6 m/sec. 

Length of the shadow on building is decreasing at 0.6 m/sec.