1.240, tells us the initial amount of algae in the lake, \frac{1}{3} tells us the common decay factor.

2.When t = 0, A\ =\ 240\ \cdot\left(\frac{1}{3}\right)^{^0}=\ 240

When t = 1, A\ =\ 240\ \cdot\left(\frac{1}{3}\right)^{^1}=\ 80

When t = 2, A\ =\ 240\ \cdot\left(\frac{1}{3}\right)^{^2}\approx26.66

When t = 3, A\ =\ 240\ \cdot\left(\frac{1}{3}\right)^{^3}\ \approx8.89

When t= 4, A\ =\ 240\ \cdot\left(\frac{1}{3}\right)^{^4}\approx2.97

3.Plugging in t = 2.5 in the equation gives us

A\ =\ 240\ \cdot\left(\frac{1}{3}\right)^{2.5}\approx15.4\ squareyards.