$$2\frac{1}{10} : 18 = \frac{2 \cdot 10 + 1}{10} : 18 = \frac{21}{10} : 18 = \frac{21}{10 \cdot 18} = \frac{21}{180} = \frac{7}{60}$$.
Ответ: $$\frac{7}{60}$$.