Final answer:
The frequency of the man doing pushups is 0.75 Hz and the period is approximately 1.33 seconds.
Step-by-step explanation:
Frequency is the number of oscillations per unit time, and it is measured in hertz (Hz), which is the number of oscillations per second. The period is the time it takes for one complete oscillation. To find the frequency of the man doing 15 pushups in 20 seconds, we divide the number of pushups by the time taken, which gives us a frequency of 0.75 Hz. The period, on the other hand, is the reciprocal of the frequency, so the period in this case is approximately 1.33 seconds.