Hi there! Recall that conversion of leaving groups specially secondary and tertiary -OH follows an SN1 mechanism. SN1 mechanism, from its name, is a unimolecular reaction. This means that the rate determining step only depends on one compound which is the Leaving group. So even if we have tons of HBr in this reaction, this will not affect the rate because recall that SN1 relies on the formation of carbocation which is achieved by de-attachment of the leaving group to the alkyl chain
So for a, the answer is doubled as well and for b, there will be now effect. Feel free to search "SN1" and "Leaving group conversion for HX"
Hope that helps!