8) 7/15 - 3/20

$$\frac{7}{15} - \frac{3}{20} = \frac{7*4}{15*4} - \frac{3*3}{20*3} = \frac{28}{60} - \frac{9}{60} = \frac{28-9}{60} = \frac{19}{60}$$
Ответ: $$\frac{19}{60}$$