| I tried it a couple times and found the pattern. Here are a couple numbers I ended up with after the subtraction. 1980 (1+9+8=18) 702 (7+2+0=9) 3915 (9+3+1+5=18) So if you give it all the numbers except the one you picked, and you don't pick a zero, then its simple. Lets say on the first one you picked the "1" you would give the computer 8 and 9 then it would figure out what number would make an answer divisible by 9 with no remainder. ie. (18/9). so it adds 8+9 to get 17 and figures out that 1 would make it divisable by 9 with no remainder.

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