A certain amount of plasticizer with a glass transition temperature $\left(T_{g}\right)$ of $-50^{\circ} \mathrm{C}$ was added to a polymer to reduce its $T_{g}$ from $70^{\circ} \mathrm{C}$ to $30^{\circ} \mathrm{C}$. Using the Fox equation, the weight fraction of the plasticizer used is $\_\_\_\_\_\_$ (rounded off to two decimal places).
