Final answer:
Thomas and Kelsey's wave has a velocity of 21 m/s, which is found by multiplying the frequency (4.2 Hz) by the wavelength (5 m).
Step-by-step explanation:
The velocity of a wave can be calculated using the formula velocity (v) = frequency (f) × wavelength (λ). In this case, Thomas and Kelsey have created a wave with a frequency of 4.2 Hz and a wavelength of 5 m. To find the velocity of their wave, we multiply the frequency by the wavelength:
v = f × λ
v = 4.2 Hz × 5 m
v = 21 m/s
Therefore, the velocity of the wave that Thomas and Kelsey produced is 21 meters per second (m/s).