729 ml of a mixture contains milk and water in the ratio 7 : 2. How much more water is to be added to get a new mixture containing milk and water in the ratio 7: 3?

Quantity of milk $=\frac{7}{9}\times 729=567\mathrm{ml}$

Quantity of water $=\frac{2}{9}\times 729=162\mathrm{ml}$

Let 'x' be the quantity that should be added to make the ratio 7:3

According the question

$\begin{array}{l}\frac{567}{162+x}=\frac{7}{3}\Rightarrow 1701=1134+7x\\ \Rightarrow 7x=1701-1134\Rightarrow x=81\mathrm{ml}\end{array}$