Question

A social security number consists of nine digits in a particular​ order, and repetition of digits is allowed. after seeing the last four digits printed on a​ receipt, if you randomly select the other​ digits, what is the probability of getting the correct social security number of the person who was given the​ receipt?

what is the formula?

Answer 1

You have to guess the first 5 digits.

You can calcualte the number of possible numbers that exist with 5 digits.

The first digit cannot be 0, it can be any from 1 to 9, so those are 9 diferent digits.

For the second, third, fourth and fifth digits there are 10 diferent posibilities for each: 0 through 9.

Then, there are 9*10*10*10*10 = 90,000 different possible social security numbers once the last four digits are known.

And the probability to get the correct social security number of the person is 1 /90,000 = 0,000011.