MathDB

By default, iPhone passcodes consist of four base-

10

digits. However, Freya decided to be unconventional and use hexadecimal (base-

16

) digits instead of base-

10

digits! (Recall that

10_{16} = 16_{10}

.) She sets her passcode such that exactly two of the hexadecimal digits are prime. How many possible passcodes could she have set?

Problem Statement