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jmc

counting and probability senior

Problem

How many positive three-digit integers less than 500 have at least two digits that are the same?
Solution
Case 1: The last two digits of our integer are equal. There are 10 possibilities for these last two digits and 4 choices for the hundred's digit, of a total of 40 possibilities. (Note that this case includes 111, 222, 333, and 444.)

Case 2: The first two digits are equal and the third is different. This occurs in ways, since we can choose the repeated digit in 4 ways and the remaining digit in 9.

Case 3: The first and third digits are equal while the second is different. This also occurs in 36 ways.

Thus we have a total of integers.

OR

Another way to solve this problem is to find how many three-digit integers less than 500 have no digits that are the same. The first digit must be 1, 2, 3, or 4. The second digit can be any of the 9 digits not yet chosen, and the third digit can be any of the 8 digits not yet chosen, so there are a total of three-digit integers that have no digits that are the same and are less than 500. There are a total of three-digit integers that are less than 500, so we have a total of integers that fit the problem. (Solution by Alcumus user chenhsi.)
Final answer
112