Length of the train = 150m, Time taken to cross a pole = 15s, Length of the platform = 200m.

Use the ratio of total distances: $T_2 = T_1 \times \frac{L_{train} + L_{platform}}{L_{train}}$. Since the train length is 150m, the platform adds 200m, so total distance becomes $150 + 200 = 350m$. Multiply the time $15s \times (350/150) = 35s$.

Students often add the length of the platform but forget to add the length of the train to the total distance, incorrectly using only the platform length in the numerator.

This concept is fundamental for 'Trains and Boats' problems; always remember that crossing a point object requires length of train, while crossing a bridge or platform requires length of train + length of object.