using 0-9, how many 4 digit sets can be formed, all digits can be repeated, ie 2576, 9471,

Question

fone company says it has used up all the digits, from an 02392 46xxxx, the first 7 numbers are set in stone but the last 4 can be any number between 0-9. my question how many uniique number variations can be formed before it runs out of combinations. they have already introduced a 02393 code how can they have used up all the numbers

Answer 1

If you mean digits can be repeated in the four digit number and all digits from 0 to 9 are available, then numbers can range from 0000 to 9999, that's 10,000 different numbers. If different digits must make up the numbers and permutations are significant, in other words, the order of the digits is important, then after picking the first digit we have 9 digits left to choose from; after picking the second digit we have 8 left to choose from, and so on. Therefore, the calculation is 10*9*8*7=5040 for 4-digit numbers.