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.

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)   (Show Source): You can put this solution on YOUR website!
in which x is how many of a year periods. Solve for x. Divide x by 3.
Answer by MathTherapy(10552)   (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:
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)   (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 (!)

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