Answer
$ 3.9608\%$ per day = $-0.039608$
Work Step by Step
Model: $ A=A_{0}e^{kt}$
Unknown: $ k $, when $ A=0.5A_{0},\ t=17.5$
$0.5A_{0}=A_{0}e^{k\cdot 17.5}\qquad... /\div A_{0}$
$0.5=e^{k\cdot 17.5}\qquad.../\ln(...)$
$-0.693147=17.5k\qquad.../\div(17.5)$
$ k\approx-0.039608$
The decay rate is
$ 3.9608\%$ per day = $-0.039608$
