Let O be the origin, O P = a and O Q = b. If R is the point on O P such that O P =5 O R, and M is the point such that OQ =5 RM, then PM is equal to:

Let $\displaystyle O$ be the origin, $\displaystyle \overrightarrow{O P}=\vec{a}$ and $\displaystyle \overrightarrow{O Q}=\vec{b}$. If $\displaystyle R$ is the point on $\displaystyle \overrightarrow{O P}$ such that $\displaystyle \overrightarrow{O P}=5 \overrightarrow{O R}$, and $\displaystyle M$ is the point such that $\displaystyle \overrightarrow{\mathrm{OQ}}=5 \overrightarrow{\mathrm{RM}}$, then $\displaystyle \overrightarrow{\mathrm{PM}}$ is equal to :