12) 1+2=

$$1\frac{3}{11}+2\frac{2}{3} = 1\frac{3 \cdot 3}{11 \cdot 3}+2\frac{2 \cdot 11}{3 \cdot 11} = 1\frac{9}{33}+2\frac{22}{33} = 3\frac{31}{33}$$

Ответ: $$3\frac{31}{33}$$