2) 3/5 - 9/20

$$\frac{3}{5} - \frac{9}{20} = \frac{3*4}{5*4} - \frac{9}{20} = \frac{12}{20} - \frac{9}{20} = \frac{12-9}{20} = \frac{3}{20}$$
Ответ: $$\frac{3}{20}$$