text: can anyone help me with this math problem? how many four digit numbers can be formed under the following condition: the leading digit cannot be zero and no repetition of digits is allowed  please explain how you got the answer u hav 0 to 9 single digit numbers. u were asked to construct a 4 digits number from those numbers.  the first digit can be either 1-9 but not zero, so u hav 9 numbers to choose.  now that u hav chosen a number let say 1  so 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.  next, again 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*  next for the last digit, u have to choose number 4-9 and 0. u have 7 choices of numbers here.  so there we go  9 x 9 x 8 x 7 = 4536 possible 4 digits numbers^^