Answer
17 mm. 17 m.
Work Step by Step
The wave speed is frequency multiplied by wavelength.
For the highest frequency:
$v = 340 \frac{m}{s} = f \lambda = (20000 Hz)(\lambda)$.
Solve for $\lambda = \frac{340 m/s}{20000 Hz} = 0.017 m$.
For the lowest frequency:
$v = 340 \frac{m}{s} = f \lambda = (20 Hz)(\lambda)$.
Solve for $\lambda = \frac{340 m/s}{20 Hz} = 17 m$.