Dear student,
We know that,
Time inetrval or time period T=1/f
where,f= frequency
Now, substituting the values in the formula
T1=1/f1
T1=1/20Hz
T1=0.05sec
T2=1/f2
T2=1/20,000 Hz
T2=0.00005 sec
Thus, the range of time period or the interval will be 0.05-0.00005 seconds.
Regards