Environment of forests following the lead of the national

Question

Question: Environment of forests following the lead of the national wildlife federation, the department of the interior of a south american country began to record an index of environmental quality that measured progress and decline in the environmental quality of its forests. The index for the years 1998 through 2008 is approximated by the function

I(t) = 1t³ - 5t² + 80            (0 ≤ t ≤ 10)
        3       2

Where t = 0 corresponds to 1998. Find the intervals where the function i is increasing and the intervals where it is decreasing. Interpret your results.