a. How many different 4 letter codes can there be?
b. What if letters cannot be repeated?
c. What if, in addition, 2 of the letters are x and y?
For a, it would simply be $26*26*26*26=456976$
For b, it would be $26*25*24*23=358800$
For c, it would be $24*23=552$
C seems a bit low to me but I think it makes sense