Let $M$ be the maximum value of the product of two positive integers when their sum is $66$. Let the sample space $S=\{\,x\in\mathbb{Z}: x(66-x)\ge \tfrac{5}{9}M\,\}$ and the event $A=\{\,x\in S:\ x\ \text{is a multiple of }3\,\}$. Then $P(A)$ is equal to: