23. 2 17/21 - 1 7/21

$$2 \frac{17}{21} - 1 \frac{7}{21} = (2-1) + (\frac{17}{21} - \frac{7}{21}) = 1 + \frac{10}{21} = 1 \frac{10}{21}$$

Ответ: $$1 \frac{10}{21}$$