\[NK = MN + \frac{4}{7} = 8\frac{5}{7} + \frac{4}{7}\]
\[NK = 8 + \frac{5}{7} + \frac{4}{7} = 8 + \frac{9}{7} = 8 + 1\frac{2}{7} = 9\frac{2}{7}\] см
\[MN + NK = 8\frac{5}{7} + 9\frac{2}{7}\]
\[MN + NK = 8 + 9 + \frac{5}{7} + \frac{2}{7} = 17 + \frac{7}{7} = 17 + 1 = 18\] см
\[MK = P - (MN + NK) = 30 - 18 = 12\] см
Ответ: 12 см