5/6 * 4/10 = ?

$$\frac{5}{6} \cdot \frac{4}{10} = \frac{5 \cdot 4}{6 \cdot 10} = \frac{20}{60}$$
Сократим дробь на 20:
$$\frac{20}{60} = \frac{20:20}{60:20} = \frac{1}{3}$$

Ответ: 1/3