5. \[\frac{5}{21} : \frac{8}{7} - \frac{1}{12} = \frac{5}{21} * \frac{7}{8} - \frac{1}{12} = \frac{5}{3*8} - \frac{1}{12} = \frac{5}{24} - \frac{2}{24} = \frac{3}{24} = \frac{1}{8}\]
Ответ: 1/8