The number of waves that pass a fixed point in a given amount of time is wave frequency. Wave frequency can be measured by counting the number of crests (high points) of waves that pass the fixed point in 1 second or some other time period. The higher the number is, the greater the frequency of the waves. :)
Zero maximum force (N) or field strength (N/C). ... minimum /maximum field strength.
Is there any options or just a type up answer?
Divide the distance by the speed:
(18 m) / (42 m/s) = 3/7 s ≈ 0.43 s