Let $f$ and $g$ be differentiabel functions on $R$ such that fog is the identity function. If for some $\mathrm{a}, \mathrm{b} \in \mathrm{R}$, $\mathrm{g}^{\prime}(\mathrm{a})=5$ and $\mathrm{g}(\mathrm{a})=\mathrm{b}$, then $\mathrm{f}^{\prime}(\mathrm{b})$ is equal to :
