Answer
$1.7$
Between 2000 and 2006, the percentage of mortgages classified as subprime increased at an average rate of around 1.7 percentage points per year.
Work Step by Step
The average rate of change of $f$ over the interval $[a, b]$ is given as:
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 $x$.
Average rate of change of $f$ over $[0,6]$ is
$\displaystyle \frac{f(6)-f(0)}{6-0}=\frac{13-3}{6}=\frac{10}{6}\approx 1.7$ percentage points per year
