Trying to help my kid with an interest rate problem...and I need help

T

tango29

Simple, but I am not explaining right or something.
Prob= $400, 7%, the problem also says the amount would double every 9.9 years(which I think is off by a bit. I figure it as over 10 years, but this is what was given).
One part of the question asks how long would it take to have $1800. I get 22.23 years, but son insists on multiplying that by the 9.9 years given for doubling and gets 220 years. I am beating my head against a rock trying to explain. Anyone could post the formula and where to plug in the numbers to get from the answer?
I think the formula is y=a(1+int)^t, where y would be the $1800, a the $400, int 75, or .07. How do I solve for t, yea I know pathetic, but having a brain fart here.
Thanks
 
Code: 
400 * 1.07^x = 1800

1800/400 = 1.07^x

4.5 = 1.07^x

log(base 1.07)4.5 = x * log(base 1.07)1.07

log(base 1.07)4.5 = x

ln 4.5/ln 1.07 = x (optional change of base step)

x = 22.23
 
Cripes, forgot about using log. Thank you, and log is part of the lesson he is working on and I completely ignored it as a way to the answer.
Man, maybe I have been out of school too long and using calculators and computers to do stuff I used to manually without a thought.
Thanks
 
LOL, too funny, as that is the site I lecture my kids to use at least weekly. My goal with my youngest is to set up a plan over the summer to get him up to speed where I know I was at his age, but he is not. I will say I will not be the most popular parent in the house, but oh well.
My oldest has been using Kahn for his chemistry at the college level. He finally listened after he was getting his butt kicked, but at least he is using it now.
What a site, that guy should get some kind of award! Just amazing.
 
Quote from tango29:

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What a site, that guy should get some kind of award! Just amazing.
More...
Bill Gate did one better, he funds him 15 mills.
 
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