What is the percentage strength $(\mathrm{v} / \mathrm{v})$ of alcohol in a mixture of $12 \mathrm{~mL}$ of $17 \% \mathrm{v} / \mathrm{v}$ alcohol, $10 \mathrm{~mL}$ of $50 \% \mathrm{v} / \mathrm{v}$ alcohol, and $500 \mathrm{~mL}$ of $33 \% \mathrm{v} / \mathrm{v}$ alcohol?