Question 94
Let $a$ and $b$ be two prime positive numbers with each other. It is said that a positive integer $n$ is weak if it cannot be written in the form $n=a x+b y$, for some nonnegative $x$ and $y$ integers. Prove that if $n$ is weak and $n\lt \frac{a b}{6}$, then there is an integer $k \geq 2$, such that $k n$ is weak.