Since we are trying to find the number of sequences can be made <em>without repetition</em>, we are going to use a combination.
The formula for combinations is:
is the total number of elements in the set
is the number of those elements you are desiring
Since there are 10 total digits, in this scenario. Since we are choosing 6 digits of the 10 for our sequence, in this scenario. Thus, we are trying to find . This can be found as shown: