a) $$\frac{2}{9} + \frac{6}{9} - \frac{3}{9} = \frac{2+6-3}{9} = \frac{5}{9}$$
б) $$8\frac{25}{27} - (3\frac{3}{27} + 2\frac{3}{27}) = 8\frac{25}{27} - 5\frac{6}{27} = 3\frac{19}{27}$$
в) $$(8\frac{3}{17} - 7\frac{15}{17}) + 3\frac{15}{17} = 8\frac{3}{17} - 7\frac{15}{17} + 3\frac{15}{17} = 8\frac{3}{17} - 4\frac{0}{17} = 4\frac{3}{17}$$
Ответ: a) $$\frac{5}{9}$$, б) $$3\frac{19}{27}$$, в) $$4\frac{3}{17}$$