20) 4 9/13-2 8/13

20) $$4 \frac{9}{13} - 2 \frac{8}{13} = (4 - 2) + (\frac{9}{13} - \frac{8}{13}) = 2 + \frac{1}{13} = 2 \frac{1}{13}$$

Ответ: 2 1/13