$\frac{1}{2} + \frac{1}{7} + \frac{1}{5} = \frac{1 * 35 + 1 * 10 + 1 * 14}{70} = \frac{59}{70}$

$\frac{5}{7} - \frac{3}{28} - \frac{1}{4} = \frac{5 * 4 - 3 - 7 * 1}{28} = \frac{20 - 3 - 7}{28} = \frac{10}{28} = \frac{5}{14}$