ขอโทษครับ เขาแก้โจทย์แล้วในเว็บ mathlinks แต่ผมลืมมาแก้ TT
Let $N$ be postive integer. Some integers are written in a blackboard. Suppose that :
1. The written number is all belong to $1,\ 2,\ \cdots N.$
2. Each of integer of $1,\ 2,\ \cdots N$ is written at least one.
3. The sum of numbers written in the black board is even.
If we mark $X$ to some numbers written and mark $Y$ to all remaining numbers, then prove that we can set the sum of numbers marked $X$ are equal to that of numbers marked $Y$.
