ANSWER:
10 m
STEP-BY-STEP EXPLANATION:
The wavelength is given by the following formula:
[tex]\begin{gathered} W=\frac{v}{f} \\ W\text{ is the wavelength} \\ v\text{ is the velocity} \\ f\text{ is the frequency} \end{gathered}[/tex]Frequency is the number of waves passing in 1 second. 4 waves passes in 2 sec, so 2 waves will pass in 1 sec. Therefore, frequency is 2 s^-1.
Replacing:
[tex]W=\frac{20}{2}=10\text{ m}[/tex]Therefore, wavelength is 10 meters
