3 31 4 42

4) $$\frac{3}{4} - \frac{31}{42} = \frac{3 \cdot 21}{4 \cdot 21} - \frac{31 \cdot 2}{42 \cdot 2} = \frac{63}{84} - \frac{62}{84} = \frac{63-62}{84} = \frac{1}{84}$$

Ответ: $$\frac{1}{84}$$