In 80 litres mixture of milk and water ratio of amount of milk to that of amount of water is 7 : 3. In order to make this ratio 2 : 1, how many litres of water should be added ?

According to the question,
$$\eqalign{ & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{Milk : Water}} \cr & {\text{Initial Ratio}}\,\,\,\,\,{{\text{7}}_{ \times 2}}:{3_{ \times 2}} \cr & {\text{Final Ratio}}\,\,\,\,\,\,\,{{\text{2}}_{ \times 7}}:{1_{ \times 7}} \cr} $$
∴ Remember water is added not milk, so make milk equal
\[\begin{gathered} \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{M}}\,\,\,\,{\text{:}}\,\,\,\,{\text{W}} \hfill \\ \left. \begin{gathered} {\text{Initial}}\,\,\,\,14\,\,\,\,:\,\,\,\,6 \hfill \\ {\text{Final}}\,\,\,\,\,\,14\,\,\,\,:\,\,\,\,7 \hfill \\ \end{gathered} \right) \hfill \\ \end{gathered} \] $$\eqalign{ & \cr & = \,20\,{\text{unit}} \cr & 1 \cr} $$
$$\eqalign{ & {\text{20 units = 80 litres}} \cr & {\text{1 unit = 4 litres}} \cr & {\text{Water added = 4 litres}} \cr} $$