Of the two primary bromides, the carbocation intermediate derived from (CH3)2CHCH2Br is more stable than that derived from CH3CH2CH2CH2Br because of greater electron donating inductive effect of (CH3)2CH group. So, (CH3)2CHCH2Br is more reactive than CH3CH2CH2CH2Br in SN1 reactions.CH3CH2CH(Br)CH3 is a secondary bromide and in SN2 reactions follows the reverse order as the steric hindrance around the electrophilic carbon increases in that order.