Решение:

1. a) $$rac{x}{7} + \frac{y}{7} = \frac{x+y}{7}$$
2. б) $$rac{m}{2} - \frac{n}{2} = \frac{m-n}{2}$$
3. в) $$rac{a}{b} + \frac{2a}{b} = \frac{a+2a}{b} = \frac{3a}{b}$$
4. г) $$rac{3x}{y} - \frac{x}{y} = \frac{3x-x}{y} = \frac{2x}{y}$$
5. д) $$rac{a+5b}{15} + \frac{2a+4b}{15} = \frac{a+5b+2a+4b}{15} = \frac{3a+9b}{15} = \frac{3(a+3b)}{3\cdot5} = \frac{a+3b}{5}$$
6. e) $$rac{b+c}{3a} - \frac{b-2c}{3a} = \frac{b+c-(b-2c)}{3a} = \frac{b+c-b+2c}{3a} = \frac{3c}{3a} = \frac{c}{a}$$
7. ж) $$rac{-3x+2y}{xy} + \frac{2y-5x}{xy} = \frac{-3x+2y+2y-5x}{xy} = \frac{-8x+4y}{xy} = \frac{4y-8x}{xy} = \frac{4(y-2x)}{xy}$$