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August 21, 2014

August 21, 2014

Posted by **Kerrie** on Sunday, April 19, 2009 at 5:20pm.

the answer is 15 but i think its 14. please explain.

- Precalc growth and decay -
**SraJMcGin**, Sunday, April 19, 2009 at 7:13pmI wonder also how the answer can be 15 years. Go to the following site, with the calculator for figuring out compound interest.

http://www.1728.com/compint.htm

Since you want to know how many years it will take:

1. click on YEARS

2. Input Principal = 500

3. Input Total = 1000

4. Input Rate = 5

I get: 14.2067!

Here's another calculator to use: http://www.moneychimp.com/calculator/compound_interest_calculator.htm

When I used 14 as the number of years = 989.97

When I used 15 as the number of years = 1039.46

You were only asked to end up with $1,000.00

Here's the formula : M = P(1+i)n

M = final amount including principal

P = principal amount

i = rate of interest per year

n - number of years invested

Most people use the compound interest calculator! Please check all your numbers carefully.

Sra

- Precalc growth and decay -
**Reiny**, Sunday, April 19, 2009 at 8:08pmSra is right.

The "calculator" on that webpage is using this formula

Amount = Principal(1+i)^n

we have 1000=500(1.05)^n

2 = 1.05^n

take log of both sides

log2 = log(1.05)^n

log2 = nlog1.05

n = log2/log1.05 = 14.2

after 14 years, your money has not yet doubled, close, but not yet.

Amount = 500(1.05)^14 = 989.97

So I guess they are right at 15, since you have to go into the 15th year to double your deposit.

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