$($a$)$ A drinking water treatment plant has $8$ tanks in parallel $($flow is split into $8$ equal streams$)$ and each tank has a volume of $80$ m$3.$ If the incoming flow to the plant is $40$ million gallons$/$day $($MGD$),$ what is the retention time in each tank? $($b$)$ If the tanks are instead operated in series $($entire flow goes first through one tank, then the second, etc$)$ what is the retention time in each tank?