We know that $70\%$ of earth's surface is covered with water out of which $97\%$ is salt water. But, $75\%$ of the fresh water available is trapped in a form of ice. So, the part of the total earth's surface that actually contains liquid fresh water is given by $\dfrac{a}{b}$, where $a$ and $b$ are coprime positive integers, find the value of $a+b$.