A 400 metre long tain crosses a 300 metre long bridge in 70 seconds. The speed of train is

Options

A. 72km/h
B. 54km/h
C. 18km/h
D. 36km/h

Answer

To solve the problem of finding the speed of the train that crosses a bridge, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the lengths involved**:
- Length of the train = 400 meters
- Length of the bridge = 300 meters

2. **Calculate the total distance covered**:
- The total distance covered when the train crosses the bridge is the sum of the lengths of the train and the bridge.
\[ \text{Total distance} = \text{Length of train} + \text{Length of bridge} = 400 \, \text{m} + 300 \, \text{m} = 700 \, \text{m} \]

3. **Identify the time taken**:
- Time taken to cross the bridge = 70 seconds

4. **Use the formula for speed**:
- Speed is calculated using the formula:
\[ \text{Speed} = \frac{\text{Distance}}{\text{Time}} \]
- Substituting the values we have:
\[ \text{Speed} = \frac{700 \, \text{m}}{70 \, \text{s}} = 10 \, \text{m/s} \]

5. **Convert the speed from meters per second to kilometers per hour**:
- To convert from m/s to km/h, we use the conversion factor \( \frac{18}{5} \):
\[ \text{Speed in km/h} = 10 \, \text{m/s} \times \frac{18}{5} = 36 \, \text{km/h} \]

6. **Conclusion**:
- The speed of the train is \( 36 \, \text{km/h} \).

### Final Answer:
The speed of the train is **36 km/h**.