Answer
There are $231$ tons of pollutant in the lake after two years.
Work Step by Step
We substitute $p_0 $ into $p_n$ in order to compute $p_1$ and then substitute $p_1 $ into $p_n$ in order to compute $p_2$ .
$p_1=(0.9) p_{1-1}+15 \\ =0.9 \ p_0+15 \\ =240$
$p_2=0.903p_{2-1}+15 \\= (0.9) (240)+15=1.03p_1+20 \\ =231$
We find that there are $231$ tons of pollutant in the lake after two years.
