Find the smallest four-digit integer divisible by three and greater than such that there are exactly two types of numbers that appear in the digits.
, 2022
Solution
We call four-digit integers good if their digits consist of exactly two types of numbers.
First, the upper two digits of integers in are . So the only good integers in the range are , , , and . But all of them are less than or equal to .
Second, the upper two digits of integers in are . So the only good integers in the range are , , , and . Since is not divisible by three, and is divisible by three, the answer is .
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