10) \(\frac{1}{ \frac{1}{35} - \frac{1}{60} }\)

$$\frac{1}{ \frac{1}{35} - \frac{1}{60} } = \frac{1}{ \frac{12}{35 \cdot 12} - \frac{7}{60 \cdot 7} } = \frac{1}{ \frac{12}{420} - \frac{7}{420} } = \frac{1}{ \frac{12 - 7}{420} } = \frac{1}{ \frac{5}{420} } = \frac{420}{5} = 84$$