1a. Вычислить: $5 \frac{8}{21} - 3 \frac{3}{21} + \frac{5}{21}$

Решение: \[ 5 \frac{8}{21} - 3 \frac{3}{21} + \frac{5}{21} = \frac{113}{21} - \frac{66}{21} + \frac{5}{21} = \frac{113 - 66 + 5}{21} = \frac{52}{21} = 2 \frac{10}{21}. \] Ответ: $2 \frac{10}{21}$.