The specific rotation of (+)-1-bromo-1-phenylethane changes from $+{15}^{°}to-{43}^{°}$ on reaction with NaOH under strict $S_{N}2$  conditions. When a sample of (–)-1-bromo-1-phenylethane was treated with aqueous NaOH, the resulting solution has a specific rotation of  $+{10}^{°}$ . The % of the reactant that underwent $S_{N}2$  reaction is