A train 280 m long, running with a speed of 63 km/hr will pass a tree in:

### Concept & Formula Crossing a tree is identical to crossing a pole; the tree has negligible length, meaning the train only needs to cover a distance equal to its own length. $$ \text{Time} = \frac{\text{Distance}}{\text{Speed}} $$ ### Step-by-Step Solution * **Given:** Length of train = $280$ m, Speed = $63$ km/hr. * **Convert Speed:** Change to m/s: $63 \times \frac{5}{18}$. * Both $63$ and $18$ are divisible by $9$, so this becomes $7 \times \frac{5}{2} = \frac{35}{2} = 17.5$ m/s. * **Calculate Time:** Time = $\frac{280}{17.5}$ * To avoid decimals, use the fraction: $\frac{280}{\frac{35}{2}} = 280 \times \frac{2}{35}$. * Since $35 \times 8 = 280$, the calculation becomes $8 \times 2 = 16$ seconds. ### Exam Strategy & Shortcut Always stick to fractions ($\frac{35}{2}$) rather than decimals ($17.5$) when dividing. It allows for quick cross-cancellation and mental math, bypassing the need for long division. ### Common Pitfall A calculation error during the conversion to m/s or relying heavily on decimals instead of easier fractions. ### Final Answer Therefore, the correct answer is **16 sec**.