Ta có: \(\overline{ab3}=\dfrac{3}{4}.\overline{3ab}\)
\(\Rightarrow\overline{ab}.10+3=\dfrac{3}{4}.\overline{ab}+300.\dfrac{3}{4}\)
\(\Rightarrow\overline{ab}.10+3=\dfrac{3}{4}.\overline{ab}+225\)
\(\Rightarrow\dfrac{40}{3}\overline{ab}=\overline{ab}+222\)
\(\Rightarrow222=\dfrac{37}{3}\overline{ab}\Rightarrow\overline{ab}=18\)