5.278 Найдите сумму: a) 7 + 2$$\frac{5}{8}$$; б) 9$$\frac{1}{9}$$ + 20; в) 64$$\frac{1}{7}$$+4$$\frac{1}{7}$$; г) 7$$\frac{5}{7}$$+4$$\frac{1}{7}$$

a) $$7 + 2\frac{5}{8} = 7 + \frac{2 \cdot 8 + 5}{8} = 7 + \frac{16+5}{8} = 7 + \frac{21}{8} = 7 + 2\frac{5}{8} = 9\frac{5}{8}$$ б) $$9\frac{1}{9} + 20 = \frac{9 \cdot 9 + 1}{9} + 20 = \frac{81+1}{9} + 20 = \frac{82}{9} + 20 = 9\frac{1}{9} + 20 = 29\frac{1}{9}$$ в) $$64\frac{1}{7}+4\frac{1}{7} = (64 + 4) + (\frac{1}{7} + \frac{1}{7}) = 68 + \frac{2}{7} = 68\frac{2}{7}$$ г) $$7\frac{5}{7}+4\frac{1}{7} = (7 + 4) + (\frac{5}{7} + \frac{1}{7}) = 11 + \frac{6}{7} = 11\frac{6}{7}$$