Math is Money

An adventure story from my time as a substitute teacher where I teach how math can make you rich.

Aug 18, 2022 3 min read

If a 15-year invested $6000 today and earned 10% per year how much would it be worth at age 65?

The answer is $704,345! That’s more than 2 orders of magnitude! Compounding interest over time is insane, but you have to start early to unlock the real value.

Back in March 2019, I did my first substitute teaching gig for fun. It was high school algebra and the topic was solving systems of equations. For the first class, I followed the teacher’s written instructions. But the kids didn’t behave and played around on their Chromebooks. I spent my break between classes throwing together a quick Excel model and started the next class with a question:

“Who is interested in me showing you how math can make you rich?” It immediately got quiet and all the hands shot into the air.

I wrote the following system of equations and told them that solving these would make them rich. The students then inundated me with questions trying to understand everything.

System of Equations to Make You Rich!

Let’s start with the 4% Rule: it’s a conservative estimate that states by withdrawing 4% of your nest egg each year it’s unlikely you’ll exhaust your savings within your lifetime. So start by asking yourself what annual income would make you happy. One of the students said he’d need $250k, but I talked them into using a more reasonable $50k. Since it was an algebra class, they knew to divide our desired annual distribution by 4% to come up with our savings goal. 50,000/.04 = $1.25 million.

That was a big number and most considered it to be unrealistic. But remember the answer to my first question above: $6k at 10% interest per annum for 50 years is $704,345. We’re already over halfway there - 56.3% to be exact. We calculate that by using the time value of money equation: FV = PV (1+r)^n Plugging in the numbers as follows: 704345 = 6000 * (1+.1)^50.

We can continue doing this for each year and only need to decrease the “n” by one. So for year two, we plug in: 6000 * (1+.1)^49 = 640,314. Boom, we’re done. 704,345 + 640,314 = 1,344,659 which is greater than our $1.25 million target. In fact, with $1,344,659 in the bank, our 4% withdrawal jumps to $53,786.

The amazing thing here is that this person wouldn’t have to invest a single dollar more, and they would be fine. But once you discover the power of compounding, it’s hard to stop. How much would they have if they continued the $6k per year until reaching age 65? Rather than do all the math here, I built a Google Sheet that lets you play around with the assumptions.1

But the answer is  $7.68 million! Bonus points for the student that wanted $250k/year because 4% of $7.68 million is $307,200. It’s also noteworthy that investing $6k for 50 years is only $300k total. And yet, one is able to withdraw more than this every single year for the rest of their life!

Some might say that it’s unrealistic for a 15-year-old to save $6k. But so what? Start with whatever is realistic. They may need to save for a few more years, but compound interest is still a powerful force. Also, is that a reasonable objection? I put together a quick thought experiment on the ‘Savings’ tab of the Google Sheet that shows how one might get to the $6k target.

Or you might also be thinking “this is great if I had known when I was 15, but I’m ‘x’ now, what should I do?” where ‘x’ is 30, 40, or 50.

I’d still recommend you get started right now to take advantage of all the compounding you can. There’s a quote of which I’m a fan:

The best time to plant a tree was 20 years ago. The second best time is today. -Chinese Proverb

I know math isn’t most people’s favorite subject. But what if we changed the name from Math 101 to Money 101?

  1. You can choose ‘File’ → ‘Make a copy’ if you have a Google account or else download an Excel version by selecting ‘File’ → ‘Download’. ↩︎

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