12 19 421+21

4. $$ \frac{12}{21} + \frac{19}{21} = \frac{12+19}{21} = \frac{31}{21}$$.

Выделим целую часть:

$$ \frac{31}{21} = 1\frac{10}{21}$$.

Ответ: $$1\frac{10}{21}$$