To solve this problem, we need to calculate the number of sound waves produced by a source of frequency 640 Hz when the sound travels a distance of 20 meters. We know the speed of sound in air is given as 320 m/s.

1. Calculate the Wavelength ($\lambda$):

   The formula for the wavelength is: $\lambda = \frac{v}{f}$ where:

   • $v$ is the speed of sound in air (320 m/s),
   • $f$ is the frequency of the sound (640 Hz).

   Substituting the values: $\lambda = \frac{320}{640} = 0.5 \text{ meters}$

2. Calculate the Number of Waves:

   To find the number of waves that fit in a distance of 20 meters, we divide the total distance by the wavelength: $\text{Number of waves} = \frac{\text{distance}}{\lambda} = \frac{20}{0.5} = 40$

Therefore, the answer is: a. 40 waves

Would you like further details or have any questions?

Here are some related questions for deeper understanding:

1. What happens to the wavelength if the speed of sound changes?
2. How does frequency affect the number of waves in a given distance?
3. What would be the number of waves if the distance was doubled?
4. How is the frequency of sound related to its pitch?
5. How can temperature affect the speed of sound and thus the wavelength?

Tip: Remember, the speed of sound is affected by environmental factors, so it may vary slightly with temperature and pressure.