probability
posted by sha .
you are to set up a code of consisting of three letters (A to Z) and five numbers (0 to 9).
a) how many codes are possible if the letters and numbers can be repeated?
b) how many codes are possible if each letter and digit can be only used once?
c) how many codes are possible if each code begins with an A and if the digits cannot be repeated?

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Sra 
B) it would be 26P4 (the number of permutations for the letters) x 9P3 (number of permutations for the digits)
You times because for each letter permutation the can be 9P3 digit permutations.