Typically, humans can hear sound waves in the frequency range 20 Hz to 20 kHz. If one wants to make digital record of sound such that no audible information is lost, what is the longest time interval between samples that could be used?
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 $f_s > 2B$ and the interval between samples $T_s = 1/f_s < 1/(2B)$. 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.