75 g of sugar solution has 30% sugar in it. Then, the quantity of sugar that should be solution to make the quantity of the sugar 70% in the solution is:

30 % sugar is in = 75 g solution

$\therefore$ 1 % sugar is in $=\frac{75}{30}\mathrm{g}$ solution

$\therefore$ 70 % sugar is in $=\frac{75}{30}\times 70 \mathrm{g}$ solution

= 175 g solution So, the quantity of sugar that should be added to the solution to make the quantity of the sugar 70 % in the solution = 175 - 75 = 100 g