20) \(\frac{3}{\frac{1}{5}+\frac{1}{10}}\)

\(\frac{1}{5}+\frac{1}{10}=\frac{2}{10}+\frac{1}{10}=\frac{3}{10}\).

\(3:\frac{3}{10}=3\cdot\frac{10}{3}=10\).

Ответ: 10