I1.1 Given that there are $a$ positive integers less than 200 and each of them has exactly three positive factors, find the value of $a$.
If $x=r s$, where $r$ and $s$ are positive integers, then the positive factors of $x$ may be $1, r, s$ and $x$. In order to have exactly three positive factors, $r=s=$ a prime number. Possible $x=4,9,25,49,121,169 . a=6$.