Решим пример:
$$10\frac{9}{8} + 1\frac{6}{8} = 10 + 1 + \frac{9}{8} + \frac{6}{8} = 11 + \frac{15}{8} = 11 + 1\frac{7}{8} = 12\frac{7}{8}$$
Ответ: $$12\frac{7}{8}$$