Singlet methylene $\text{:}\mathrm{CH}{}_2$ inserts into C-H bonds of 2-methylpropane, $(\mathrm{CH}{}_3){}_3\mathrm{CH}$. Assume that the insertion is completely unselective, with every C-H bond equally likely to react. What percentage of the product mixture is 2-methylbutane?

2-Methylpropane has 9 primary C-H bonds (three $\mathrm{CH}{}_3$ groups) and 1 tertiary C-H bond. Insertion into a primary C-H gives $(\mathrm{CH}{}_3){}_2\mathrm{CHCH}{}_2\mathrm{CH}{}_3$, 2-methylbutane, and insertion into the tertiary C-H gives 2,2-dimethylpropane. With equal probability per bond, 2-methylbutane is $9/(9+1)\times 100=90\%$.