How many 3 digit numbers have at least one repeated digit?

Answer 1

There are a total of 900 3-digt numbers (100 thru 999);

Of these there are 9x9x8 = 648 numbers that do not contain

repeating digits (first position: 9 possibilities, second place 9 possibilities;

the 10 digits minus the one used in the first place. and third place 8 possibilities;

10 digits minus the two used in the first and second places)

So, there should be 900-648=252 3-digit numbers with

at least one repeating digit.