Answer
The index was decreasing at an average rate of $200$ points per day.
Work Step by Step
The average rate of change of $f$ over the interval $[a, b]$ is
Average rate of change of $f\displaystyle \ \ =\frac{f(b)-f(a)}{b-a}$
The units of the average rate of change of $f$ are units of $f(x)$ per unit of $x$.
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Using the formula above, we get:
$\displaystyle \frac{I(5)-I(2)}{5-2}=\frac{1000-1600}{3}=-200$ (points per day)
The index was decreasing at an average rate of $200$ points per day.
