Solve for the last number... 8809=6 7111=0 2172=0 6666=4 1111=0 3213=0 7662=2 9313=1 0000=4 2222=0 3333=0 5555=0 8193=3 8096=5 7777=0 9999=4 7756=1 6855=3 9881=5 5531=0 2581=?

That's not officially it bob, but maybe you found a pattern that I was not aware of. Explain it, I don't know what you mean. Do you mean the sum of the four digits that are equal to zero?

Remove 1 2 3 5 7 because 1111=0 2222=0 3333=0 5555=0 7777=0 Now you have 8809=6 7111=0 2172=0 6666=4 3213=0 7662=2 9313=1 0000=4 8193=3 8096=5 9999=4 7756=1 6855=3 9881=5 5531=0 Now remove them again 6666=4 66=2 9=1 0000=4 89=3 8096=5 9999=4 6=1 68=3 988=5 See the solution? 6,9,0 are 1's 8's are 2's

I'm getting lost in your solution bob... probably cuz my final project is frying my brain... but the solution I have is MUCH simpler.

Your solution is the same. I did it by assuming the numbers were codes. I probably over thought the puzzle but when you do them you must first guess and then test or eliminate possibilities. I saw patterns and made assumptions. I actually got it quicker because the answer was already posted. Assume the numbers mean something - but what. Use the info to see which can simplify the problem and remove them - then look for the pattern. Edit. I see puzzles as functions with a domain and a range.

Oh wait... I think I see how you arrived at the same thing I did... After a minute of not seeing what I would naturally look for, I suddenly saw the answer.