28. \(\frac{1}{\frac{1}{35}+\frac{1}{60}}\)

\(\frac{1}{35}+\frac{1}{60}=\frac{12}{420}+\frac{7}{420}=\frac{19}{420}\).

\(1:\frac{19}{420}=\frac{420}{19}=22\frac{2}{19}\).

Ответ: \(\frac{420}{19}\).