3) (6 3/4 - 5 1/8) : 2 1/5;

$$ (6\frac{3}{4} - 5 \frac{1}{8}) : 2\frac{1}{5} = (6\frac{6}{8} - 5\frac{1}{8}) : 2\frac{1}{5} = 1\frac{5}{8} : 2\frac{1}{5} = \frac{13}{8} : \frac{11}{5} = \frac{13}{8} \cdot \frac{5}{11} = \frac{13 \cdot 5}{8 \cdot 11} = \frac{65}{88} $$.

Ответ: $$\frac{65}{88}$$