Выразим $$b$$:
$$b = 12\frac{1}{6} - 4\frac{8}{15}$$
$$b = \frac{73}{6} - \frac{68}{15} = \frac{73 \cdot 5}{6 \cdot 5} - \frac{68 \cdot 2}{15 \cdot 2} = \frac{365}{30} - \frac{136}{30} = \frac{229}{30} = 7\frac{19}{30}$$
Ответ: $$b = 7\frac{19}{30}$$