Milk contains 5% water. What quantity of pure milk should be added to 10 liters of milk reduce this  to 2%?

Initially  $5\%$ is the portion of  the water in the mixture, 95% is  the portion of the milk in the mixture.  So quantity of  water in litres = 0.5 and also quality of  milk in $10L=\frac{95}{100}\times 10=9.5L$

Let  x be the a variety of  pure milk to be added to the mixture to make water 2%.

$\frac{0.5}{9.5+x}\times 100=\frac{0.2}{9.8}\times 100\Rightarrow 9.8\times 0.5=0.2(9.5+x)$

$\Rightarrow x=15$