20) 12 * (1/12 + 1/6)

$$12 \cdot (\frac{1}{12} + \frac{1}{6}) = 12 \cdot (\frac{1}{12} + \frac{1 \cdot 2}{6 \cdot 2}) = 12 \cdot (\frac{1}{12} + \frac{2}{12}) = 12 \cdot \frac{1+2}{12} = 12 \cdot \frac{3}{12} = 12 \cdot \frac{1}{4} = \frac{12}{4} = 3$$
Ответ: 3