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In how many positive four-digit integers that are not multiples of $1111$ do the digits form an arithmetic sequence? (The digits must form an arithmetic sequence, in order. For example, the number 5137 does not count.)

 Sep 10, 2020
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I can only think of 9 such numbers:

 

1234  1357  2345  2468  3456  3579  4567  5678  6789  Total =  9

 Sep 10, 2020

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