The removal of substance A is a first$-$order reaction. In single CSTR with a volume of $10$ m$3$ and a flow rate of $10$ m$3/$hr$,$ the removal of substance A is $90\%.$

a$)$ If a boffle is placed in the middle of the reactor, what will be the removal efficiency in $\%?$

b$)$ In the case of option a$,$ what should be the flow rate to keep the efficiency at $90\%?$