Here’s a quick puzzle that looks almost too easy: A box contains 900 balls, and 90% of them are red. How many are not red? Most people can answer immediately, and this time, the obvious answer really is correct. If 90% are red, then the remaining 10% must be something other than red.

Now calculate 10% of 900. Divide 900 by 10, and you get 90. You can also check the answer another way: 90% of 900 is 810 red balls, and 900 minus 810 leaves 90 balls that are not red.

The wording may tempt some puzzle-solvers to search for a hidden trick. We aren’t told what colors the remaining balls are, but that doesn’t affect the calculation. If 90% of the balls are red, exactly 10% are not red. The idea that all 900 could somehow be red would contradict the original statement, because then 100% would be red rather than 90%.

So there’s really only one mathematically correct answer: 90 balls are not red. Did you get it right away, or did you start looking for a trick? Sometimes the easiest puzzles are the ones that make us question the answer simply because they seem too obvious.