# rate of change question I need some explanation

I tried to solve them myself but couldn't; the questions look kind of different from the explanation first given. Any help on how to approach this problems would be much appreciated. thank you a lot!

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Hint: If you have $Q(t)=ke^{ct}$ and it takes $4$ years to triple, you are given $Q(4)=3Q(0)$. Inserting the expression for $Q$ gives $ke^{4c}=3k$ Now solve this for $c$ and find the $t'$ such that $Q(t')=2Q(0)=2k$

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Thank you Ross for the explanation. –  Muath05 Mar 18 '13 at 18:34
C=Ln(3)/4 is it right? –  Muath05 Mar 18 '13 at 18:35
as for t': Ke^ct' = 2K (the 3 K cancel out)==> e^[ln3/4]t' =2 ==> t' = 4ln2/ln3.is it right? –  Muath05 Mar 18 '13 at 18:40
@Muath05: you got it. In your second comment you use C instead of c. They are not the same. –  Ross Millikan Mar 18 '13 at 18:42
sorry I meant c. thanks for noticing it. any idea about the first problem, please? –  Muath05 Mar 18 '13 at 18:47