Решение:

1. $$7\frac{5}{6} - 3\frac{2}{3} = 7\frac{5}{6} - 3\frac{4}{6} = (7-3) + (\frac{5}{6} - \frac{4}{6}) = 4 + \frac{1}{6} = 4\frac{1}{6}$$
2. $$8\frac{5}{12} - 6\frac{7}{20} = 8\frac{25}{60} - 6\frac{21}{60} = (8-6) + (\frac{25}{60} - \frac{21}{60}) = 2 + \frac{4}{60} = 2\frac{1}{15}$$
3. $$11\frac{11}{12} - 5\frac{7}{9} = 11\frac{33}{36} - 5\frac{28}{36} = (11-5) + (\frac{33}{36} - \frac{28}{36}) = 6 + \frac{5}{36} = 6\frac{5}{36}$$
4. $$9\frac{17}{24} - 8\frac{11}{36} = 9\frac{51}{72} - 8\frac{22}{72} = (9-8) + (\frac{51}{72} - \frac{22}{72}) = 1 + \frac{29}{72} = 1\frac{29}{72}$$