The original puzzle
If Mary's brother has one brother more than he has sisters, how many brothers more than sisters does Mary have ?
This one is simple but not obvious. A lot of adults miss it.
The problem lies in the abstraction of the language,
with its incessant array of "brother" and "sister".
The trick is to name the characters -
1. Call Mary's brother George.
2. George has one sister (Mary) and therefore 2 brothers (call them Peter and Paul).
3. The family consists of one girl (Mary) and three boys (George, Peter, and Paul).
4. Mary has no sisters and three brothers (George, Peter, and Paul).
and we have the answer.
Most people, including adults answer "two".