A man standing at `51 m` from a wall fires a gun. Calculate the time after which an echo is heard. The speed of sound is `340 m//s`.

Explanation

To solve the problem of calculating the time after which an echo is heard when a man fires a gun 51 meters away from a wall, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Scenario**:
- A man is standing 51 meters away from a wall and fires a gun. The sound travels to the wall and then reflects back to the man, creating an echo.

2. **Calculate the Total Distance Traveled by Sound**:
- The sound travels to the wall and back, so the total distance (D) is twice the distance to the wall:
\[ D = 2 \times 51 \, \text{m} = 102 \, \text{m} \]

3. **Use the Speed of Sound**:
- The speed of sound (v) is given as 340 m/s.

4. **Apply the Formula for Time**:
- The time (t) taken for the sound to travel this distance can be calculated using the formula:
\[ t = \frac{D}{v} \]
- Substituting the values we have:
\[ t = \frac{102 \, \text{m}}{340 \, \text{m/s}} \]

5. **Calculate the Time**:
- Performing the division:
\[ t = \frac{102}{340} \approx 0.3 \, \text{s} \]

6. **Conclusion**:
- The time after which the echo is heard is approximately 0.3 seconds.

### Final Answer:
The echo will be heard after approximately **0.3 seconds**.

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