interest math
posted by lynda on .
doris browning deposited $8000 into a savings account paying 5.25% interest. how long will it take for her investment to grow to $10,100. The answer i got was 4yr 11mo 30 days but that is wrong someone please help

I am assuming you are using the standard compound interest method, simple interest is seldom used for periods longer than a year
8000(1.0525)^n = 10100
1.0525^n = 1.2625
n log 1.0525 = log 1.2625
n = 4.5554 years , assuming the rate is 5.25 per annum compounded annually
at continuous compounding
8000 e^(.0525n) = 10100
e^(.0525n) = 1.2625
take ln of both sides
.0525n = ln 1.2625
n = 4.4399 years
btw:
at simple interest
8000 + 8000(.0525)n = 10100
n = 5 years
none of the 3 ways I used produces your answer, I have no idea how you got that. 
I belive its 5 years.( asuming the interest rate is anual)
8000x .0525 = 420
(Check[8000/420*5=100])
so 101008000 = 2100
Finally 2100/420 = 5
Hope that's right.