в) $$18 \cdot \frac{5}{21} - \frac{2}{3}$$;

в) $$18 \cdot \frac{5}{21} - \frac{2}{3} = \frac{18 \cdot 5}{21} - \frac{2}{3} = \frac{6 \cdot 5}{7} - \frac{2}{3} = \frac{30}{7} - \frac{2}{3} = \frac{30 \cdot 3}{7 \cdot 3} - \frac{2 \cdot 7}{3 \cdot 7} = \frac{90}{21} - \frac{14}{21} = \frac{76}{21} = 3\frac{13}{21}$$

Ответ: $$3\frac{13}{21}$$