180 Days of Ideas for Discussion in Math Class.
(as of 9July2014, we're in overtime!)
Should that last number be 8?
The 7 is correct.
Like Dan is alluding to, I also thought it was subtract the two numbers to get the new number. But then I noted the "error" at the end.So then I looked solely at the "13 & 21 -> 7." First thought that popped into my head was add the digits of the two numbers and you get 7. Tried that out from the first set of numbers, and it worked. Checked with the rest (determining that ? = 12) and saw that it all worked out.
WTF! Nice catch, Jason. Sharp little dagger there, Curmudgeon.
Wish I could take credit for it, but I wanted people to try it before telling that it's one of Nob Yoshigahara's puzzles. There's another rendition of it at http://www.puzzles.com/puzzleplayground/NobsNumberTree/NobsNumberTree.htm