$\frac{3}{4} + \frac{1}{2} = \frac{3}{4} + \frac{2}{4} = \frac{5}{4} = 1\frac{1}{4}$;
$1\frac{1}{4} + \frac{1}{8} = 1\frac{2}{8} + \frac{1}{8} = 1\frac{3}{8}$;
$1\frac{3}{8} - 1\frac{1}{4} = 1\frac{3}{8} - 1\frac{2}{8} = \frac{1}{8}$.

$\frac{3}{5} - \frac{2}{15} = \frac{7}{15}$;
$\frac{3}{5} - \frac{3}{20} = \frac{9}{20}$;
$\frac{3}{5} + \frac{3}{10} = \frac{9}{10}$.