Problem - 1119
Prove: randomly select $51$ numbers from $1$, $2$, $3$, $\dots$, $100$, there must exist two numbers for which one is a multiple of the other.
Express these $100$ numbers as $2^k \cdot j$, where $j=1$, $3$, $5$, $\dots$, $99$, and then group all the numbers by $j$:
{1, 2, 4, 8, 16, 32, 64}
{3, 6, 12, 24, 48,96}
{5, 10, 20, 40, 80}
...
{99}
There are totally $50$ such sets. By the pigeonhole principle, at least two chosen numbers will be in the same set. By construction of these sets, these two numbers must meet the requirement.