When the alkyl bromides (listed here) were subjected to hydrolysis in a mixture of ethanol and water

Question:

When the alkyl bromides (listed here) were subjected to hydrolysis in a mixture of ethanol and water (80% EtOH/20% H2O) at 55oC, the rates of the reaction showed the following order:
(CH3)3CBr > CH3Br > CH3CH2Br > (CH3)2CHBr
Provide an explanation for this order of reactivity.