$\mathop {C{H_3} - \mathop {CH}\limits_{\mathop {|\,\,\,\,\,}\limits_{Br\,\,} } - C{H_2} - C{H_3}}\limits_{\scriptstyle{\rm{2}} - {\rm{Bromo}}\,{\rm{butane}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{ }}\atop \scriptstyle\,\,\,{\rm{(Main}}\,{\rm{product)}}} + \mathop {C{H_3} - C{H_2} - C{H_2} - C{H_2} - Br}\limits_{\scriptstyle{\rm{1}} - {\rm{Bromo}}\,{\rm{butane}}\atop \scriptstyle\,\,\,\,\,\,\,\,\,\,\,{\rm{(Minor)}}} $

$2-$ Bromobutane is the main product because

${2^o}$ carbonium ion is more stable than ${1^o}$.