Chapter 5 - Number Theory and the Real Number System - 5.3 The Rational Numbers - Exercise Set 5.3 - Page 285: 78

$\dfrac{8}{15}$

To combine fractions, make sure that the fractions have the same denominators before combining their numerators. \begin{align} \dfrac{ 1 }{ 3 } + \dfrac{ 1 }{ 5 } &= \dfrac{ 5 }{ 15 } + \dfrac{ 3 }{ 15 } &&\text{Make equivalent fractions} \\ &= \dfrac{ 5 + 3 }{ 15 } &&\text{Add the numerators} \\ &= \boxed{ \dfrac{8}{15} } \end{align}