This situation is represented by an exponential function since it increases double every 4 hours
It has an initial value of 90 pounds of algae:
[tex]\begin{gathered} 12\text{ hours have 3 periods of 4 hours, then} \\ \text{Exponential function is represented:} \\ y=ax^n \\ a=\text{initial value} \\ x=\text{common ratio} \\ n=\text{period} \\ \end{gathered}[/tex]So,
[tex]\begin{gathered} y=90(2)^3 \\ y=90(2)^3 \\ y=720\text{ pounds} \end{gathered}[/tex]