Please, somebody, explain how this works to me before my head explodes: worked everytime so far I always KNEW my computer could read my mind.
Try calculating your answer with a different starting number a few times (before clicking on the sphere). You will get the same sign no matter what.
This has been posted before. The secret is, no matter what 2 digit number you choose, the end result will always be a factor of 9 (i.e. the answer will always be 9, 18, 27, 36, 45, 54, 63, 72, 81 or 90). If you look at the chart, all those numbers have the same symbol.