(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

How many three digit numbers are there that the middle digit is the average of the first and last digit.

3. The attempt at a solution

I started with 9 because of 111, 222,...999

then I added 7 for 123, 234,...789

then 5 for 135, 246,...579

3 for 147, 258, 369

1 for 159

that gives 24 but I doubled it for 123 and 321

that's 48 but I then subtracted 9 for 111=111

that leaves 39 which is not one of the possible correct answers.

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# Homework Help: Number problem

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