# 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 $Q(t')=2Q(0)=2k$