The rate $($in mg carbon$/$m$3/$h$)$ at which photosynthesis takes place for a species of phytoplankton is modeled by the function

P $=$

$80$I$/$I$^2+$ I $+4$

where I is the light intensity $($measured in thousands of foot$-$candles$).$ For what light intensity is P a maximum?

I $=$ thousand foot$-$candles