Challenge

Write down the numbers 1,2,...,2n for some positive integer n. Now out of these choose n+1 numbers. Prove that out of these n+1 numbers there exists numbers a and b such that a divides b.

demonstration/example.
1,2,3,4  n=2 so n+1=3.
The only set that doesn't contain 1 is {2,3,4} and 2|4.

1, 2.

|{1 / 3 = 4.}|

|Q.E.D.}