Решение:

a) $$18 - 3\frac{5}{6} = 18 - \frac{3 \cdot 6 + 5}{6} = 18 - \frac{18+5}{6} = 18 - \frac{23}{6} = 18 - 3\frac{5}{6} = 17\frac{6}{6} - 3\frac{5}{6} = (17 - 3) + (\frac{6}{6} - \frac{5}{6}) = 14 + \frac{1}{6} = 14\frac{1}{6}$$

Ответ: $$14\frac{1}{6}$$

б) $$6\frac{3}{16} \cdot 4 = \frac{6 \cdot 16 + 3}{16} \cdot 4 = \frac{96 + 3}{16} \cdot 4 = \frac{99}{16} \cdot 4 = \frac{99 \cdot 4}{16} = \frac{99}{4} = 24\frac{3}{4}$$

Ответ: $$24\frac{3}{4}$$