Answer
$ 10\%$
Work Step by Step
Reading the graph,
in 1950, PV= 500 (thousands)
in $1990$ (t=$40$ years), FV=$2500$ thousands
$INT=PVrt$
$FV-PV=PVrt\qquad $... solve for r ...
$r=\displaystyle \frac{FV-PV}{PV\cdot t}=\frac{2500-500}{500\cdot 40}=0.1$,
or
$ 10\%$
