ARGH! Need maths help!
Mar. 8th, 2010 08:01 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png)
I was once good at maths.
I know I should know how to do this, but I cannot remember enough to see if my method is effective or not. I found a website that will let me punch in numbers and give me an answer, but I want to check it! So I am hoping that
shocolate or someone similarly gifted is up and about.
I start with $50. Every week, I add $50. I have a compounding interest rate of 9.96%. I compound it annually, or monthly (two results). What do I have at the end of 21 years?
More than happy to do all the actual working if someone can remind me of what the formulae are.
I know I should know how to do this, but I cannot remember enough to see if my method is effective or not. I found a website that will let me punch in numbers and give me an answer, but I want to check it! So I am hoping that
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I start with $50. Every week, I add $50. I have a compounding interest rate of 9.96%. I compound it annually, or monthly (two results). What do I have at the end of 21 years?
More than happy to do all the actual working if someone can remind me of what the formulae are.
no subject
Date: 2010-03-08 09:44 am (UTC)http://en.wikipedia.org/wiki/Compound_interest
Sorry I can't be more help...I've never been any good at this type of maths. Give me triple differential equations and I'm fine. Interest rates and anything using real nulbers instead of symbols...eeeep! No can do!
no subject
Date: 2010-03-08 09:56 am (UTC)It shouldn't be that hard ... but I can't do it!
Really it's:
50*52*21 (the amount per week, times weeks, times years), plus
1.0083*50*251 (monthly interest rate, times first month's money, times all months bar the first, plus
(1.0083 squared)*50*250, plus
(1.0083 cubed)*50*249, plus ...
And I KNOW I can work out how to do this, if only I could remember how!
Weeks in 21 years
Date: 2010-03-08 10:18 am (UTC)The formulas here (which was suggested by
Ewen
Re: Weeks in 21 years
Date: 2010-03-08 11:29 am (UTC)Back to sherryillk's formula! It wouldn't work for me for m>c, but maybe it will if m=c!
I knew I would regret not practicing my maths as much as my French and Italian one day!
Interest
Date: 2010-03-08 09:45 am (UTC)Note that IME most savings accounts are actual daily compounding, with interest credited 1-4 times a year, rather than only compounding once or twice a year. But given your hypothetical interest rate (9.96%! does anyone pay even half that these days?!) perhaps you have a hypothetical account too that really does only compound once or twice a year.
My brute force perl script suggests that with weekly compounding (irrespective of when it's credited), you'd have over $185,000 at the end (with exactly how much depending a bit on the details of leap years you happen to cross, and hence number of weeks involved). This calculator suggests you'd have a bit over $174,000 if the interest were only compounded annually (alas it doesn't have a biannual compounding option, only daily/monthly/quarterly, but the figure ought to be somewhere in between those two values -- or more precisely between the quarterly and yearly figures).
I hope that helps,
Ewen
PS: Of those amounts about $55,000 is weekly cash contributions and the rest is interest. (1.0996)^21 is nearly 7.5 (so approx 7500%), but obviously not all of the cash is there at the start to get interest, so figures in the $170,000-$185,000 range are quite believable.
Re: Interest
Date: 2010-03-08 09:54 am (UTC)Ewen
Re: Interest
Date: 2010-03-08 10:02 am (UTC)The last time I did anything like this, I proved that you would have been about $10 billion richer if you followed one particular financial adviser for 30 years. Of course, he was the little columnist who turned up for work on the train and wore a nice hat but old jacket, because he could only be happy investing in things he didn't know anyone on the board of ...
no subject
Date: 2010-03-08 09:58 am (UTC)It seems to be along the lines you are looking for... But it seems to yield a pretty bad-ass looking equation that makes me want to run for the hills... >
no subject
Date: 2010-03-08 12:37 pm (UTC)no subject
Date: 2010-03-08 01:20 pm (UTC)Anyway, I went and tried both scenarios and I got $196,083.16 when it's compounded once annually and $184,053.28 when it's compounded monthly. I tried to be as accurate as I can but it's hard when the only calculator you have is your computer one... >< It really made me wish I had my graphing calculator but alas, it's been lent out to my brother...
no subject
Date: 2010-03-08 01:24 pm (UTC)It's too late, I am off to write fic!
no subject
Date: 2010-03-08 01:38 pm (UTC)no subject
Date: 2010-03-08 12:04 pm (UTC)having a maths degree doesn't seem to help in the Real World...
no subject
Date: 2010-03-08 12:38 pm (UTC)no subject
Date: 2010-03-08 10:05 pm (UTC)no subject
Date: 2010-03-08 10:07 pm (UTC)no subject
Date: 2010-03-08 10:12 pm (UTC)no subject
Date: 2010-03-08 12:30 pm (UTC)By year:
Firstly, we calculate the amt of savings in a year = $50 X 4 weeks X 12 months = $2400
Total savings at the end of 1st yr
= 2400 + (9.66/100)(2400)
= 2400(1.0966)
Total savings at the end of 2nd yr
= (2400 + 2400(1.0966))(1.0966)
= 2400 (1.0966 + 1.0966^2)
Total savings at the end of 3nd yr
= (2400 + 2400(1.0966) + 2400(1.0966)^2)(1.0966)
= 2400 (1.0966 + 1.0966^2 + 1.0966^3)
.
.
.
Total savings at the end of 21st yr
= 2400 (1.0966 + 1.0966^2 + 1.0966^3 + ... + 1.0966^21)
= 2400 (1.0966 [1.0966^21 -1]/[1.0966 -1]
= 161682.8889
(using the summation formula of geometric progression, Sn = a(1-r^n)/(1-r))
Similar for month...
I have no idea how accurate this is. It's what I learn in secondary school (and I'm now doing applied math and not finance lol)
no subject
Date: 2010-03-08 12:32 pm (UTC)no subject
Date: 2010-03-08 12:36 pm (UTC)*Weeps at yet another area of stupidity ...*
no subject
Date: 2010-03-08 12:41 pm (UTC)amt of savings in a month = $50 X 4 weeks = $200
Total savings at the end of 1st month
= 200 + (9.66/100)(200)
= 200(1.0966)
... and so on for 21X12 months?
Or did I get your question completely wrong?
Compounded monthly on a principle increased weekly... erm, sorry does it mean you +50 weekly and the interest per month is 9.96%?
(this is what happens when you learn high school maths in Malay)
no subject
Date: 2010-03-08 12:48 pm (UTC)no subject
Date: 2010-03-08 12:58 pm (UTC)no subject
Date: 2010-03-08 01:06 pm (UTC)no subject
Date: 2010-03-08 01:10 pm (UTC)no subject
Date: 2010-03-08 01:09 pm (UTC)no subject
Date: 2010-03-08 01:11 pm (UTC)no subject
Date: 2010-03-08 01:14 pm (UTC)no subject
Date: 2010-03-08 01:26 pm (UTC)I might be able to work it out using Excel.
no subject
Date: 2010-03-08 01:31 pm (UTC)no subject
Date: 2010-03-08 01:34 pm (UTC)All the same if he can't pull himself away from the spaceships let me know.
no subject
Date: 2010-03-08 09:52 pm (UTC)no subject
Date: 2010-03-08 09:56 pm (UTC)no subject
Date: 2010-03-08 09:57 pm (UTC)no subject
Date: 2010-03-09 12:22 am (UTC)Balance = P(1+i)^n + c/i[(1+i)^n-1]
P=principal, i=interest/period, n=# periods, c=contribution/period
You've probably figured it all out by now, though :)
no subject
Date: 2010-03-09 06:01 am (UTC)no subject
Date: 2010-03-09 09:22 pm (UTC)