To sum two fractions, you have to transform them in order to have the same denominator: if you multiply both numerator and denominator of the first fraction by 8, and numerator and denominator of the second by 3, you have
[tex] \dfrac{2}{3} = \dfrac{2\cdot8}{3\cdot8} = \dfrac{16}{24} [/tex]
[tex] \dfrac{3}{8} = \dfrac{3\cdot3}{8\cdot3} = \dfrac{9}{24} [/tex]
Now we can easily sum the fractions, because we only have to sum the numerators:
[tex] \dfrac{2}{3} + \dfrac{3}{8} = \dfrac{16}{24} + \dfrac{9}{24} = \dfrac{16+9}{24} = \dfrac{25}{24} [/tex]
