A little boy buys 12 tomatoes. On the way home, most of them get ruined. All but nine. How many are left in good condition?
Most people immediately subtract. 12 minus 9 equals 3. They go with 3 and feel completely confident. They are wrong.
Solution:
The answer is 9.
"All but nine" doesn't mean all minus nine. It means all except nine. Nine tomatoes survived. The rest got ruined. The phrasing is the entire trick.
This is one of those riddles where reading slowly is the only skill you need. The math was never the problem. The words were.
Did you fall for it?