3-Methylbut-1-ene reacts with HBr (no peroxide). The major product is

Protonation of $\mathrm{CH_2=CH-CH(CH_3)_2}$ at the terminal carbon gives the secondary cation $\mathrm{CH_3-\overset{+}{C}H-CH(CH_3)_2}$ (Markovnikov). A 1,2-hydride shift converts it into the more stable tertiary cation $\mathrm{CH_3CH_2-\overset{+}{C}(CH_3)_2}$, which is captured by $\mathrm{Br^-}$ to give 2-bromo-2-methylbutane as the major product; 2-bromo-3-methylbutane is the minor unrearranged product.