3 1 20 50 -- --

$$\frac{3}{20} - \frac{1}{50} = \frac{3 \cdot 5}{20 \cdot 5} - \frac{1 \cdot 2}{50 \cdot 2} = \frac{15}{100} - \frac{2}{100} = \frac{13}{100}$$

Ответ: $$\frac{13}{100}$$