20) -31+81 55

$$ -3\frac{1}{5} + 8\frac{1}{5} = -(3 + \frac{1}{5}) + 8 + \frac{1}{5} = -3 - \frac{1}{5} + 8 + \frac{1}{5} = 8 - 3 + (\frac{1}{5} - \frac{1}{5}) = 5 + 0 = 5 $$

Ответ: 5