1) $bc = \frac{3}{8} * \frac{3}{2} = \frac{9}{16}$;
2) $c = bc : b = \frac{9}{21} : \frac{3}{4} = \frac{3}{7} * \frac{4}{3} = \frac{1}{7} * \frac{4}{1} = \frac{4}{7}$;
3) $b = bc : c = \frac{16}{21} : \frac{2}{3} = \frac{16}{21} * \frac{3}{2} = \frac{8}{7} * \frac{1}{1} = 1\frac{1}{7}$.

1) $b : c = \frac{5}{6} : \frac{4}{9} = \frac{5}{6} * \frac{9}{4} = \frac{5}{2} * \frac{3}{4} = \frac{15}{8} = 1\frac{7}{8}$;
2) $c = b : (b : c) = \frac{7}{10} : \frac{1}{2} = \frac{7}{10} * \frac{2}{1} = \frac{7}{5} * \frac{1}{1} = 1\frac{2}{5}$;
3) $b = (b : c) * c = \frac{2}{5} * \frac{5}{4} = \frac{1}{1} * \frac{1}{2} = \frac{1}{2}$.