He is correct, that the value 16 given to MID$ as a 'start' will yield an "F" because the 'start' argument to the MID$ function is one-based.

This is the string:

     A$ = "0123456789ABCDEF"

And calling the function will give you: MID$ (A$,1,1) => "0" ... MID$ (A$,15,1) => "E" MID$ (A$,16,1) => "F"

MID$ (