Uncertainty Relation between time and frequency
Sampling a signal creates its spectrum replica as Nyquist Shannon theorem stated. As a consequence, to avoid aliasing, one must sample at a rate greater than twice the max frequency $B$ of the signal spectrum. Otherwise, you have aliasing as illustrated in the Figure below.



In your case, $B = 20$kHz hence sampling rate $f_s &gt; 2B$ and the interval between samples $T_s = 1/f_s &lt; 1/(2B)$. You can see that $T_s$ is upperbounded by $1/(2B) = 50$ microseconds.

Note that the longest $T_s$ is not 50 microseconds but $(50-\epsilon)$ microseconds  with $\epsilon$ is arbitrarily small positive quantity.