Question

why there are 10000 possible four-digit codes for a pin when digits are 0-9.

Answer 1

For a four digit pin with digits 0-9, we can have repeated numbers too.

So each digit of the pin can have any number from 0-9, which means each digit have 10 ways.

So the total number of 4 digit pins

= 10*10*10*10= 10,000.

This is the case where repetition of numbers is allowed.

In other terms we can say that the number of intergs from 0000 to 9999 = 10000. In either way the answer is same.

Please feel free to ask your doubts in the comment section.Thank you.