Posted by **A.J.** on Sunday, November 29, 2009 at 9:16pm.

Jason and Mary have a new baby. They want to invest $20,000 at the baby’s birth in order to have $100,000 for the baby’s college education. What interest rate do they need to earn in order to achieve this in 18 years?

Use the following formula to determine the average interest rate:

r = (S/P)^1/n – 1

where S = the final amount

P = the initial investment

n = number of years

- Algebra -
**Damon**, Sunday, November 29, 2009 at 9:47pm
yes. S = P (1+r)^n

100 = 20 (1+r)^18

(1+r)^18 = 5

18 log (1+r) = log 5

using base 10 logs

18 log(1+r) = .699

log(1+r) = .0388

take antilog

1+r = 1.0935

r = .0935

times 100 = 9.35 %

- Algebra -
**A.J.**, Sunday, November 29, 2009 at 9:55pm
Thanks for the help but I still don't understand. What does log and antilog stand for?

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