Monochlorination of 2-methylpropane (isobutane) is carried out, taking the relative reactivity per hydrogen as $1^{\circ }:3^{\circ }=1:5$. The percentage of the tertiary chloride, 2-chloro-2-methylpropane, in the product mixture is

Isobutane has 9 primary H and 1 tertiary H. Weighted yields are $9\times 1=9$ and $1\times 5=5$. The tertiary chloride is $5/14\times 100=35.7$ per cent. The statistical factor (9 against 1) outweighs the modest 5-fold preference, so the primary chloride is actually the major product.