Need fast don't understand both a and b show work

Problem A
Usually the number of bits in a byte is 8 or 16 or 32 and recently 64. You don't have to write a formula to restrict it to this number of bits. You are not asked to do so. The general formula is 2^n - 1 for the problem of Millie and her golden keys. Somehow the system can be made to choose the right number of bits. Apple IIe s for example, used 8 bits and there was a location that told the processor that fact.

2^n - 1 <<<<< Answer

Problem B
In this case n = 4
2^n - 1 = 2^4 - 1 = 16 - 1 = 15

Millie can collect 15 keys <<<<<< Answer