The volume of oxygen liberated from 15 mL of 20 volume H₂O₂, is

20 volume of hydrogen peroxide indicates that 20 L of oxygen gas produce from 1 L or 1000 mL of hydrogen peroxide. Volume of oxygen produce by 15 mL of hydrogen peroxide is calculated as:

$\begin{array}{ccc}1000 \mathrm{mL} {\mathrm{H}}_2{\mathrm{O}}_2& =& 20 \mathrm{L} {\mathrm{O}}_2\\ 15 \mathrm{mL} {\mathrm{H}}_2{\mathrm{O}}_2& =& 15 \mathrm{mL} {\mathrm{H}}_2{\mathrm{O}}_2\times \frac{20 \mathrm{L} {\mathrm{O}}_2}{1000 \mathrm{mL} {\mathrm{H}}_2{\mathrm{O}}_2}\\ & =& 0.3 \mathrm{L} {\mathrm{O}}_2\\ & =& 300 \mathrm{mL} {\mathrm{O}}_2\end{array}$