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Thread: Interest rate help?

  1. #1
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    Interest rate help?

    Hello all, I am confused on where to go with this problem after a certain point.

    Question- What interest rate--to at least one decimal place--compounded weekly do you need to earn to make $8000 double in 9 years?

    So far I have done this but not sure if it is correct-

    r=8000(1+r/52)^52*9

    16000/8000=8000(1+r/52)^52*9

    2=8000(1+r/52)^52*9

    Any help would be greatly appreciated. Thank you!
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  2. #2
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    Re: Interest rate help?

    Why have you put r on the LHS? It's the final amount that should go there. The rule is

    $\displaystyle \begin{align*} A &= P\left( 1 + R \right) ^n \\ 16\,000 &= 8000 \left( 1 + \frac{r}{52} \right) ^{52 \cdot 9 } \end{align*}$

    Go from here...
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