Over the course of 10 years, someone puts $400 per month into an investment account that earns 6% per year (compounds monthly). What is the value of the account at the end of 10 years?

1 Answer

$65, 879. 50


Let's work this out month by month for a few months, find the pattern, then solve.

We're putting $400 per month into an account that is earning 6% per year with the interest compounding monthly.

Let's work out the interest rate first: we have #(6%)/"year"#, which gives us #(6%)/(12 "months")# or #1/2%# per month.

We start month 1 by adding $400 to our account. We earn interest over the month, so have #400 xx (1+.005) = 402#.

In month 2, we add $400 to the account, #402 + 400=802# and at the end of the month we have another round of interest: #(802) xx (1+.005) = 806.01#

Is there a pattern yet?


Let's do one more and I think it will be clear:

In month 3, we add $400 to the account, #806.01 + 400=1206.01# and at the end of the month we have another round of interest: #(1206.01) xx (1+.005) = 1212.04#

Let's look at how we got here:


This is #sum_(n=1)^120((400)(1+.005))^n#

and far easier to compute on a spreadsheet or a financial calculator.

The answer I came up with on my spreadsheet is $65, 879. 50. I also worked it out on an online financial calculator and it returned $65, 551. 74 - which is what my sheet returned at the start of Month 120 but I calculated it through with that last month's interest (and so I think my answer is correct).

Here is where that financial calculator that I mentioned is located: