\[\frac{a + 5b}{15} + \frac{2a + 4b}{15} =\]
\[= \frac{a + 5b + 2a + 4b}{15} = \frac{3a + 9b}{15} =\]
\[= \frac{3(a + 3b)}{15} = \frac{a + 3b}{5}.\]