text: u hav 0 to 9 single digit numbers.\nu were asked to construct a 4 digits number from those numbers.\n\nthe first digit can be either 1-9 but not zero, so u hav 9 numbers to choose.\n\nnow that u hav chosen a number let say 1\n\nso next, u have to choose another number to be the second digit. because u were asked for no repetation, your only choice is number 2-9 and 0 so u hav another 9 numbers to choose from. so let say u choose number 2. so we hav now 12**, two more digits to fill in.\n\nnext,\nagain the rule no repetation, so for the third digit u can choose any number between 3-9 and 0. u have now 8 numbers to choose from. let say u choose number 3. so u have 123*\n\nnext for the last digit, u have to choose number 4-9 and 0. u have 7 choices of numbers here.\n\nso there we go\n\n9 X 9 X 8 X 7 = 4536 possible 4 digits numbers^^