23. \(\frac{1}{\frac{1}{30}+\frac{1}{42}}\)

\(\frac{1}{30}+\frac{1}{42}=\frac{7+5}{210}=\frac{2}{35}\).

\(1:\frac{2}{35}=\frac{35}{2}=17,5\).

Ответ: 17,5.