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I'm not taking this class but I was going through the text book and ran into an issue. The problem states:

If you invest a dollar at "6% interest compounded monthly," it amounts to (1.005)

^{n}dollars after*n*months. If you invest $10 at the beginning of each month for 10 years (120 months), how much will you have at the end of the 10 years?Now, the problem expects you to use the formula for partial sum of a geometric series.

[tex]S_n=\frac{a(1-r^n)}{1-r}[/tex]

So, as far as I can tell, the series is

[tex]10*1.005+10*1.005^2+10*1.005^3+...[/tex]

which would mean:

a = 10*1.005 = 10.05,

r = 1.005,

and n = 120.

r = 1.005,

and n = 120.

Computing S

_{n}with those values gives about $1646.99.

Interestingly, Quora gave the exact same answer: https://www.quora.com/Investing-que...much-will-you-have-at-the-end-of-the-10-years

My problem? Compound interest calculators give a totally different number. For example:

http://www.bankrate.com/calculators/savings/compound-savings-calculator-tool.aspx

gives $2014.

screencap: https://s15.postimg.org/gxaujjt8b/compoundinterest1.jpg

https://www.investor.gov/additional...l-planning-tools/compound-interest-calculator

gives $2013.46.

screencap: https://s17.postimg.org/4ai0gb7xb/compoundinterest2.jpg

So, I am kind of lost here. Did I do the problem wrong? If so please enlighten me. I double checked their value of 1.005 using (1+r/n)nt; (1 + 0.06/12) = 1.005, so 1.005

^{n}appears to be valid to me.

Any suggestions?