How many 3 digits can be formed when at least one digit is 5

Dear Dhriti,

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.

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i think 310