Therefore,
(CH3)2CHCH2Br is more reactive than CH3CH2CH2CH2Br in SN1 reactions CH3CH2CH(Br)CH3 is a secondary bromide and (CH3)3CBr  is a tertiary
bromide Hence the above order is followed in SN1 The reactivity in SN2
reactions follows the reverse order as the steric hinderance around the
electrophilic carbon increases in that order