Each of the following bromides is treated with $\mathrm{NaI}$ in dry acetone at $25^{\circ }\mathrm{C}$. Which of them react faster than bromoethane under these conditions?

Methyl bromide is less hindered than ethyl bromide. Allylic and benzylic bromides are activated because the SN2 transition state is stabilised by overlap with the adjacent $\pi$ system. Isopropyl bromide is secondary and slower than the primary ethyl bromide.