A rubber contains $70 \mathrm{wt} \%$ butadiene (molar mass $=54 \mathrm{~g} / \mathrm{mol})$, $20 \: \mathrm{wt} \%$ isoprene (molar mass $=68 \mathrm{~g} / \mathrm{mol})$, $5 \: \mathrm{wt} \%$ sulfur and $5 \: \mathrm{wt} \%$ carbon black. Assume that all the sulfur is present in crosslinks.
If each sulfide crosslink contains an average of two sulfur atoms, the percentage of possible crosslinks that are joined by vulcanization is $\_\_\_\_\_ \: \%$. (Round off to one decimal place)
