The alkane shown is monochlorinated with $\mathrm{Cl}{}_2$ and light, and the whole chloride mixture is then heated with alcoholic $\mathrm{KOH}$ (E2, every available beta hydrogen counted). How many different alkenes (constitutional isomers) can be obtained in total?

The alkane is 2-methylbutane, which gives four chlorides. Dehydrochlorination gives 2-methylbut-1-ene (from 1-chloro-2-methyl and 2-chloro-2-methyl), 2-methylbut-2-ene (from the tertiary and the 2-chloro-3-methyl chloride) and 3-methylbut-1-ene (from 1-chloro-3-methyl and 2-chloro-3-methyl). Only three distinct alkenes result.