a zip-code is any 5-digit number, where each digit is an integer 0 through 9. for example, 92122 and 00877 are both zip-codes. how many zip-codes have exactly 3 different digits?

A zip-code is any 5-digit number, where each digit is an integer 0 through 9.  There are 67,500 zip codes with exactly 3 different digits.

To find the number of 5-digit zip codes with exactly 3 different digits, we can break the problem down into cases based on the number of each type of digit.

Case 1: One digit is repeated 2 times, and the other 3 digits are distinct.

There are 10 choices for the repeated digit, and ${5 \choose 2}$ ways to choose the positions for the repeated digits. For each choice of repeated digit, there are $9 \times 8$ ways to choose the distinct digits, and $3!$ ways to arrange them. Therefore, the total number of zip codes in this case is:

10⋅( 5/2)⋅9⋅8⋅6 = 54,720

Case 2: One digit is repeated 3 times, and the other 2 digits are distinct.

There are 10 choices for the repeated digit, and ${5 \choose 3}$ ways to choose the positions for the repeated digits. For each choice of repeated digit, there are $9$ ways to choose the distinct digit, and $2!$ ways to arrange them. Therefore, the total number of zip codes in this case is:

10(5/3)⋅9⋅2=2,700

Case 3: Two digits are repeated, each one twice, and the remaining digit is distinct.

There are ${10 \choose 2}$ ways to choose the repeated digits, and ${5 \choose 2}$ ways to choose the positions for the first repeated digit. Once the positions for the first repeated digit are chosen, the positions for the second repeated digit are determined. There are 8 choices for the distinct digit. Therefore, the total number of zip codes in this case is:

(10/2)*(5/2)*8=10,080

Adding up the zip codes from each case, we get a total of:

54,720+ 2,700+ 10,080= 67,500

Therefore, there are 67,500 zip codes with exactly 3 different digits.

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