Решение:

а) $$3,8 + (-8,9) = 3,8 - 8,9 = -5,1$$

б) $$-3,4 + 2,5 = -0,9$$

в) $$-\frac{2}{3} + \frac{5}{6} = -\frac{2 \cdot 2}{3 \cdot 2} + \frac{5}{6} = -\frac{4}{6} + \frac{5}{6} = \frac{1}{6}$$

г) $$1\frac{2}{7} + (-\frac{5}{7}) = 1\frac{2}{7} - \frac{5}{7} = \frac{9}{7} - \frac{5}{7} = \frac{4}{7}$$

д) $$-1\frac{3}{8} + \frac{5}{8} = -\frac{11}{8} + \frac{5}{8} = -\frac{6}{8} = -\frac{3}{4}$$

е) $$4 + (-3\frac{5}{6}) = 4 - 3\frac{5}{6} = 4 - \frac{23}{6} = \frac{4 \cdot 6}{6} - \frac{23}{6} = \frac{24}{6} - \frac{23}{6} = \frac{1}{6}$$