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Math Help - Compounding quarterly!

  1. #1
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    Compounding quarterly!

    find the time required for an investment of 5000 dollars to grow to 9000 dollars at an interest rate of 7.5 percent per year, compounded quarterly.

    t= _____

    this is what i have got so far please correct me if i am wrong

    9000=5000(1+.075/4)^4t

    so you divide by 5000 on each side leaving you with
    9000/5000= 1.8


    1.8= (1+.075/4)^4t

    Ln1.8 = Ln(1+.075/4)^4t

    divide by 4(1+.075/4)


    ln1.8/4ln(1+.075/4) = t

    then whats next? cuz i got that this above part equals 7.9

    so then i replugged it back into the first equation but it says its not correct

    8993.04676

    used 7.9 ??
    Last edited by evinez; April 22nd 2008 at 12:03 PM. Reason: got part of answer
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  2. #2
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    Quote Originally Posted by evinez View Post
    find the time required for an investment of 5000 dollars to grow to 9000 dollars at an interest rate of 7.5 percent per year, compounded quarterly.

    t= _____

    this is what i have got so far please correct me if i am wrong

    9000=5000(1+.075/4)^4t

    so you divide by 5000 on each side leaving you with
    9000/5000= 1.8


    1.8= (1+.075/4)^4t

    Ln1.8 = Ln(1+.075/4)^4t

    divide by 4(1+.075/4)


      \ln1.8/{\color{red}(} 4\ln(1+.075/4)/{\color{red})} = t

    then whats next? cuz i got that this above part equals -.584
    If you use brackets then you'll get t =7.9
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