Answer
$k=-0.0068$
Work Step by Step
$A=A_{0}e^{kt}$
For $A_{0}=7.1, A=5.4, t=40$
$5.4=7.1e^{40k}$
$0.761=e^{40k}$
$\ln 0.761=40k$
$k=-0.0068$
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