The major product of the reaction of${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CHBrCH}}_3$ with alcoholic KOH is

This reaction is governed by Saytzeff's rule. According to this rule, the elimination of β-hydrogen atom takes place from the carbon having the smaller number of H-atoms or in other words, a stable alkene is formed (More substituted alkene is more stable)

CH₃CH(Br)CH₂CH₃ $\stackrel{\mathrm{Alc}.\mathrm{KOH}}{\to }$CH₃CH=CH−CH₃ (major product) + CH₂=CHCH₂CH₃ (minor product)