Discussion

A train crosses a platform of 120 m in 15 sec, same train crosses another platform of length 180 m in 18 sec. then find the length of the train?

Answer: B

Each crossing covers the train plus that platform.

\[ L + 120 = 15v, \qquad L + 180 = 18v \]

Subtracting:

\[ 60 = 3v \quad\Longrightarrow\quad v = 20 \text{ m/s} \]

\[ L = 15 \times 20 - 120 = 180 \text{ m} \]

Subtraction removes the unknown length in a single step, leaving the speed.

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