Answer
$ 1.25\%$
Work Step by Step
For simple interest,
$INT=PVrt$,
$FV-PV=PVrt$,
$FV=PVrt+PV$,
PV is constant and r is constant, so
$FV$, as a function of time, is a linear function
(with PV as y-intercept and $PV\cdot r$ as slope).
From $ FV=5t+400\qquad$ we read
$ PV=400\qquad$ (y-intercept) and
$ 400\cdot r=5\qquad$ (slope) ... divide by 400,
$r=\displaystyle \frac{5}{400}=$0.0125
or
$ 1.25\%$
