Hi, my college professor gave me this brain teaser that I've been working at for a week but to no avail. Hopefully someone here can give me some assistance, because my professor won't. Each natural number can be put into one of two categories: correct and incorrect. Each number will always be either incorrect or correct and will never change from one to the other. Here are the examples he gave us: (note that the list of numbers is never-ending, he just gave us a small list) Correct: 1-10, 12, 18, 20, 21, 24, 27, 30 Incorrect: 11, 13-17, 19, 22, 23, 25, 26, 28, 29 Over the past week, I've gotten to tell me at least this: there is no pattern, no sequence. Each number can be looked at individually and can be categorized within a few seconds, though longer numbers take a little longer to figure out (i.e. 4135). No calculator is necessary to do the math. ... To categorize each number, one needs to perform TWO math operations, that is +, -, x, or division. No complex math or big numbers involved. A sixth grader has the math knowledge to solve this. The result you get from doing these two operations will give you a number that has a certain number property. The property is what tells you which category it gets put into. Good luck!