A train running at a certain speed crosses a stationary engine in 20 seconds. To find out the speed of the train, which of the following information is necessary?

### Concept & Distance Formula When a train crosses a stationary object that has a significant length (like a platform, bridge, or another engine), the total distance covered is the sum of the lengths of both the train and the object. $$Speed = \frac{Length\ of\ Train + Length\ of\ Object}{Time\ Taken}$$ ### Step-by-Step Solution * **Given:** The time taken to cross the stationary engine = 20 seconds. * Let the length of the train be $L_1$ and the length of the stationary engine be $L_2$. * The total distance to be covered by the train to completely clear the engine = $L_1 + L_2$. * Using the speed formula: $Speed = \frac{L_1 + L_2}{20}$ * Since the formula requires both $L_1$ and $L_2$ to calculate a numerical value for the speed, we need both the length of the train and the length of the engine. ### Exam Strategy & Shortcut For problems involving a train crossing a non-point stationary object, the "distance" is always the sum of both lengths. If either length is missing, you cannot find the speed. You can immediately mark "both are needed" without writing down the equation. ### Common Pitfall A frequent error is treating the stationary engine like a pole or a standing man, assuming its length is zero. Always check if the crossed object has a measurable length. ### Final Answer Therefore, the correct answer is **Both the length of the train and the length of the engine**.