



 
Hi Nancy, Differentiate $f(t) = e^{0.02t}  e^{0.42t},$ set the derivative equal to zero and simplify. You should be able to reach an expression of the form \[e^{At} = B e^{Ct}.\] Take the natural log of both sides to get \[A t = \ln(B) + C t.\] Solve for $t.$ Penny  


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