Ответ:

\[\frac{1}{24}+\frac{1}{56}=\frac{7}{168}+\frac{3}{168}=\frac{10}{168}=\frac{5}{84}.\]

\[\frac{5}{84}:\frac{20}{21}=\frac{5}{84}\cdot\frac{21}{20}=\frac{1}{16}.\]

Ответ: \(\frac{1}{16}\).