Math
posted by Jassie .
How many codes consisting of a letter followed by 3 digits can be made if no digit can be used more than once?

26*10*9*8
This assumes the letter "O" is allowed. Often it is not, because of confusion with zero. IT also assumes case of the letters is indifferent. If case masters, then A and a will be different, so you double the number of codes available. 
can you tell me how you got 9 and 8?
I get it 26 alphabets.
10 digits.