Don't Let It Go Away!

A $6\times 4$ grid contains $24$ small squares each of side length $5$ . A coin of diameter $4$ is thrown on the grid such that it does not cross the outer boundary. The probability that the coin stays inside a small square, that is, doesn't cross an inner boundary line can be represented as $\dfrac{a}{b}$ where $\gcd(a,b)=1$ . Find $a+b$ .