how many codes can you make with A B C and D if no letter can be repeated?

Answer 1

For the first digit of the code, you have 4 choices

For the second digit since there are none that repeat, you have 3 choices.

For the third digit, there are 2 choices

For the fourth digit, there is only 1 choice left.

This would be:

4 x 3 x 2 x 1 = 24 codes