My three-digit code is 023. Reckha can't choose a code that is the same as mine in two or more of the three digit-positions. Reckha can otherwise choose any three-digit code where each digit is in the set . How many codes are available for Reckha?
Which digits are in "the set"?
If the set consists of all ten digits, then there are 1000 possible combinations.
Of these 30 are not allowed: 02x, 0x3, x23 (where x is any of the ten digits.
In this case there are 1000 - 30 allowable combinations.