Решение:

1. $$\frac{a}{4} + \frac{b}{8} = \frac{2a}{8} + \frac{b}{8} = \frac{2a+b}{8}$$
2. $$\frac{n}{10} - \frac{m}{5} = \frac{n}{10} - \frac{2m}{10} = \frac{n-2m}{10}$$
3. $$\frac{6}{25} + \frac{a}{100} = \frac{24}{100} + \frac{a}{100} = \frac{24+a}{100}$$
4. $$\frac{1}{2a} + \frac{1}{2b} = \frac{b}{2ab} + \frac{a}{2ab} = \frac{b+a}{2ab}$$