In three trials with a stopwatch, a student counts the following numbers of crests passing a point on a rope:
Trial 1: 50 crests in 20 s
Trial 2: 52 crests in 19 s
Trial 3: 49 crests in 21 s
(a) Calculate the frequency for each trial.
(b) Determine the average frequency across the three trials.
(c) Find the corresponding average time period.