Subset

How many subsets with three elements can be formed from the set $\{1,2,3,\cdots ,20\}$uch that $4$ is a factor of the product of the three numbers in the subset?

Prime

Let $x$ be an integer of the form $$x=\underset{ntimes}{\underbrace{111\cdots 11}}$$ Show that, if $x$ is prime then $n$ is prime.