How many 4 digits numbers can be formed using 0 1 2 3 4 5 6 7 8 9?

Answer 1

We figure out how many possibilities exist for each digit, and multiply them all together. Think of the digits as slots that need to be filled: _ _ _ _ The first digit can be anything but 0; 0345 is not a 4-digit number, for example. So there are 9 choices for the first digit. All the subsequent digits can be anything from 0-9; there are 10 choices for digits two, three, and four. So there are 9x10x10x10 = 9000 possible numbers with 4 digits.