Time Taken to meet/catch = $\dfrac{\text{Initial distance separating them}}{\text{Relative Speed}}$
$\text{Relative Speed =} S_1 \pm S_2$
$\text{Relative Speed =} S_1 + S_2 \text{ ,when Both are moving towards opposite direction}$
$\text{Relative Speed =} S_1 - S_2 \text{ ,when Both are moving towards same direction}$
$∴\color{green}{\text{Time taken to meet/catch = }}$$\color{blue}{\dfrac{6\hspace{0.1cm} km}{(6+12)\hspace{0.1cm}kmph} = \dfrac{6}{18}\hspace{0.1cm}hr = \dfrac{1}{3} \hspace{0.1cm}hr = \dfrac{1}{3}\times 60 \hspace{0.1cm}min} =\color{orange}{ 20\hspace{0.1cm} min}$