Akeycode must contain 4 numerals. there are 9 numerals available. using these numerals , how many different keycodes may be created update : (answer is : 3,024)

From the answer, let's assume that you can only use each number once.
So there are four slots.

_ _ _ _

For the first slot, you have a possibility of 9 numbers.

9 _ _ _

Now for the second slot, you have the possibility of 8 numbers.

9 8 _ _

Now for the third slot, you have the possibility of 7 numbers.

9 8 7 _

Now for the last slot, you have the possibility of 6 numbers.

9 8 7 6

Now multiply these numbers together and you should get 3,024.