TeX source:
\frac{2}{3}+\frac{7}{8} = \frac{2}{3}\,\cdot\frac{8}{8} + \frac{7}{8}\,\cdot\frac{3}{3} = \frac{16}{24}+\frac{21}{24} = \frac{16+21}{24} = \frac{35}{24}