The speed of sound is calculated using:
\[\nv = f \times \lambda\n\]
Where:
- \( f = 500 \, \text{Hz} \) (frequency)
- \( \lambda = 80 \, \text{cm} = 0.8 \, \text{m} \) (wavelength)
Substituting the values:
\[\nv = 500 \times 0.8 = 400 \, \text{m/s}\n\]
Time \( t \) to travel 1 km:
\[\nt = \frac{\text{distance}}{\text{speed}} = \frac{1000}{400} = 2.5 \, \text{seconds}\n\]
Therefore, the answer is 2.5 seconds.