Tính:

\[\frac{3}{7} + \frac{9}{7} = \frac{{3 + 9}}{7} = \frac{{12}}{7}\]

\(\frac{7}{8} - \frac{5}{8} = \frac{{7 - 5}}{8} = \frac{2}{8} = \frac{{2:2}}{{8:2}} = \frac{1}{4}\)

\[\frac{{13}}{{45}} + \frac{{25}}{{45}} = \frac{{13 + 25}}{{45}} = \frac{{38}}{{45}}\] 

\(\frac{3}{{15}} - \frac{1}{{15}} = \frac{{3 - 1}}{{15}} = \frac{2}{{15}}\)