1.a wave has a period of 20 seconds. what is the frequency?

2.in 4 seconds 3 waves pass a point. what is the frequency? what is the period?

3.what is the speed of a wave that has a frequency of 2hz, an amplitude of 10m, and a wave length of 5m?

1).  Frequency = (1/period) = 1/(20sec)  =  (1/20) per sec  .

2).  Frequency= (3 waves) / (4 sec) = (3/4) (per sec)
       Period  =  (1/freq)  =  1/(3/4 per sec) = (4/3) sec

3). Speed= (freq) x (wavelength)  (2 per sec) x (5 m)  =  (2 x 5) ( m per sec)
     Speed doesn't depend on amplitude.