“Hexadecimal uses digits that more closely resemble our usual base-10 counting system and it's therefore easier to decide at a glance how big a number like e7 is as opposed to 11100111. Higher information density. With 2 hexadecimal digits, we can express any number from 0 to 255.”
Lowercase a is decimal 97 ; upper case is decimal 65
It's easier to think of them in octal, however: a = octal 141, and A is octal 101
octal to binary is easy, each digit is three bits.
141 = 001 100 001
101 = 001 000 001
So, how many bits are changed above?