A boy invests a sum of $4000 on his 10th birthday in the year 1996, with an annual interest rate of 7%, compounded every six months. During some stage of the investment, the interest rate changes to an annual rate of 6%, still compounded every six months. On the boy's 18th birthday, the boy checks his account to see a sum of $6770. At what point in time did the interest rate change from 7% to 6%?

(In reply to analytic solution by Charlie)

I stand corrected.  My data was correct. My analysis wasn't.  Age 15.5 looks good for the time of the rate decrease.

Posted by Leming on 2006-10-04 12:20:54