Five identical plates are connected across a battery as in figure. If the charge on plate $1$ be $+q$, then the charges on the plates $2,3,4$ and $5$ are:

The plate $2$ is the common plate between $1$ and $3$ and it is connected to negative end of battery. So its charge should be $-(q+q)=-2q$.
Similarly plate $3$ is in between $2$ and $4$ but it is connected to positive end of battery. So its charge $=+2q$ and plate $4$ is in between $3$ and $5$ and it is connected to negative end of battery. So its charge should be $-(q+q)=-2q$.
Plate $5$ is single plate and it is connected to positive end of battery. So its charge $=+q$