Answer
$=\dfrac {1}{3}$
Work Step by Step
$0.\overline {3}=0.3333...=\dfrac {3}{10}+\dfrac {3}{10^{2}}+\dfrac {3}{10^{3}}-..=\dfrac {a_{1}}{1-r}=\dfrac {\dfrac {3}{10}}{1-\dfrac {1}{10}}=\dfrac {\dfrac {3}{10}}{\dfrac {9}{10}}=\dfrac {3}{9}=\dfrac {1}{3}$