20) 12 * (7/12 + 5/6)

$$
12 \cdot \left(\frac{7}{12} + \frac{5}{6}\right) = 12 \cdot \left(\frac{7}{12} + \frac{5 \cdot 2}{6 \cdot 2}\right) = 12 \cdot \left(\frac{7}{12} + \frac{10}{12}\right) = 12 \cdot \frac{17}{12} = 17
$$

Ответ: 17