An oscillating body (Pendulum)
Makes 1 amplitude in 0.02 seconds
What is its frequency?
Not clear (to me!) exactly what you mean. Do you mean one period is 0.02s, or the time between the mid-line and the maximum (or minimum) is 0.02s, in which case the period would be 4*0.02s or 0.08s (or do you mean something else?)? Either way the frequency is given by frequency = 2pi/period.
The periodic time is 0.08 second. (I found it by 4*0.02 as you did)
Frequency = 1/periodic time
1/0.08 = 12.5HZ Is that correct ??
1 Amplitude = 1/4 periodic time = 1/4 complete oscillation
so 4 amplitudes = periodic time of 1 complete oscillation