Step 1:  Understand the question and note down the given data

Step 2: Prove that the two triangles are similar by using the given data.

            EXPLANATION: From the figure we get

           \angle B = \angle Q = 90 \degree

           \angle C = \angle R

            Because, AC || PR, all sun’s rays are parallel at any instance)

          So,  ∆ABC ~ ∆PQR ( by AAA Theorem)

Step 3: Use the AAA theorem to calculate unknown value

        AAA THEOREM: In two triangles, if the angles are equal, then the sides

        opposite to the equal angles are in the same ratio (or proportional) and hence

        the two triangles are similar. 

                   \frac{AB}{PQ} = \frac{BC}{QR}

Step 4: Substitute the known values in the AAA theorem

                  EXAMPLE:   \frac{1.65}{PQ} = \frac{1.8}{5.4}

                                       PQ = \frac{5.4 * 1.65}{1.8}

                                                  PQ = 4.95 m

                          The height of the lamp post is 4.95 m