When 10 mL of ethyl alcohol (density = 0.7893 g ml^-1) is mixed with 20 mL of water (density 0.9971 g mL^-1) at 25^oC, the final solution has a density of 0.9571 g mL^-1. The percentage change in total volume on mixing is

Total weight of alcohol and water

$=10\times 0.7893+20\times 0.9971$

$\begin{array}{l}\text{ Volume of mixture }=\frac{10\times 0.7893+20\times 0.9971}{0.9571}\\ =29.08 \mathrm{mL}\\ \begin{array}{lc}\text{ Change in volume }=(20+10)-29.08& \\ =0.92 \mathrm{mL}& \\ \mathrm{\%}\text{ change in volume }=\frac{0.92\times 100}{30}\approx 3.06\%& \\ \approx 3.1\mathrm{\%}& \end{array}\end{array}$