9x10x10x10x10x10
1. 9 digits can go on 1st place (0,1,2,3,4,5,6,7,8) 9 possible combinations
2. 10 digits can go on 2nd place (0,1,2,3,4,5,6,7,8,9) 10 possible combinations
3. 10 digits can go on 3td place (0,1,2,3,4,5,6,7,8,9) 10 possible combinations
4. 10 digits can go on 4th place (0,1,2,3,4,5,6,7,8,9) 10 possible combinations
5. 10 digits can go on 5th place (0,1,2,3,4,5,6,7,8,9) 10 possible combinations
6. 10 digits can go on 6th place (0,1,2,3,4,5,6,7,8,9) 10 possible combinations
You multiple all possible combinations to get the total number of combinations.
The result will be different if you are not allowed to replace the digit after it is picked.
1. 9 digits can go on 1st place (0,1,2,3,4,5,6,7,8) 9 possible combinations
2. 9 digits can go on 2nd place (0,1,2,3,4,5,6,7,8,9 minus the picked from previous step(s)) 9 possible combinations
3. 8 digits can go on 3td place (0,1,2,3,4,5,6,7,8,9 minus the picked from previous step(s)) 8 possible combinations
4. 7 digits can go on 4th place (0,1,2,3,4,5,6,7,8,9 minus the picked from previous step(s)) 7 possible combinations
5. 6 digits can go on 5th place (0,1,2,3,4,5,6,7,8,9 minus the picked from previous step(s)) 6 possible combinations
6. 5 digits can go on 2nd place (0,1,2,3,4,5,6,7,8,9 minus the picked from previous step(s)) 5 possible combinations
tottal = 9x9x8x7x6x5 = 136080