SOLUTION: How long will it take for $600 to double if it is invested at 9% annual interest compounded 3times a year? Enter in exact calculations or round to 3 decimal places.

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Question 1160547: How long will it take for $600 to double if it is invested at 9% annual interest compounded 3times a year? Enter in exact calculations or round to 3 decimal places.
Found 3 solutions by josgarithmetic, MathTherapy, ikleyn:
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
600%2A%281.03%29%5Ex=1200 in which x is how many 1%2F3 of a year periods. Solve for x. Divide x by 3.

Answer by MathTherapy(10552) About Me  (Show Source):
You can put this solution on YOUR website!
How long will it take for $600 to double if it is invested at 9% annual interest compounded 3times a year? Enter in exact calculations or round to 3 decimal places.
For DOUBLING, you go with the following formula: matrix%281%2C3%2C+2%2C+%22=%22%2C+%281+%2B+i%2Fm%29%5E%28mt%29%29

                           In this case, we get: 

                       Time taken to double, or 



You can check by applying the Rule of 72.

Since the interest rate of 9% is between 6% and 10%, the RULE of 72 serves as a GREAT estimate of the time it takes for an invested amount to DOUBLE.

This gives us:   

               

As seen, the RULE of 72 estimates that it'll take about 8 years for an investment to DOUBLE, at a 9% ANNUAL interest rate.

Answer by ikleyn(52781) About Me  (Show Source):
You can put this solution on YOUR website!
.

Since the compounding is  3  times per year  (which means  every  4  months),
the time period  MUST  BE  a multiple of  4  months.

So the final and  UNIQUELY  correct answer is  8  (eight,  EIGHT)  FULL  YEARS,

and there is  NOTHING  TO  ROUND  in this problem.

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Notice  (AND  IT  IS  EXTREMELY  IMPORTANT)  that in this problem the instruction
to round to  3  decimal points  DOES  NOT  MAKE  any sense.


This instruction is written by the person who didn't think on the problem any single minute (!)