SLot method

[sfbay]

how many 4 digit codes possible using digits 0-9.
restriction first number can not be 0.

is it as simple as: 9 x 9 x 8 x 7

[jnelson0612]

how many 4 digit codes possible using digits 0-9.
restriction first number can not be 0.

is it as simple as: 9 x 9 x 8 x 7

If our only restriction is that the first number cannot be 0, we have:

9 possibilities for the first number (1-9)
10 possibilities for the second, third, and fourth number (0-9)

So that is 9 * 10 * 10 * 10 = 9,000

[sfbay]

additional restriction is using digit only once

[jnelson0612]

additional restriction is using digit only once

In that case I agree with your original equation.