3) 12 8/15+8 9/15

$$12 \frac{8}{15}+8 \frac{9}{15} = (12+8) + (\frac{8}{15} + \frac{9}{15}) = 20 + \frac{8+9}{15} = 20 + \frac{17}{15} = 20 + 1 \frac{2}{15} = 21 \frac{2}{15}$$.

Ответ: $$21 \frac{2}{15}$$