The Doubling Trick
How long does it take for money to double?
There is a shortcut you can do on paper. Take 72 and divide it by how much something grows each year. If it grows by 6 out of every 100, that is 72 ÷ 6 = 12 years to double. The 6 here is a made-up number — pick any one you like and the shortcut still works. It is arithmetic, not a promise about any particular investment.
Here's a trick you can do on paper.
Take 72. Divide it by how fast something grows.
Grows by 6 each year? 72 ÷ 6 = 12 years to double.
Grows by 9? Then it's 8 years. Same trick.
That's it. No calculator, no app.
Go deeper — try this yourself
Do the division on paper.
Write 72 at the top of a page and pick your own divisor. Doing the division by hand makes the answer feel like yours. If you pick 7 and get 10.3 years, that's fine — a messy answer is more convincing than a clean one, because it looks like real math instead of a party trick somebody handed you.
Flag the made-up number, out loud.
Say it out loud, every time: “I'm making this number up so I have something to divide.” It sounds small. It isn't. People who practice arithmetic with a fake rate attached to a real-sounding promise grow up believing projections. Naming your own assumption, every single time, is the actual habit this trick is teaching you.
Test it on a huge number.
Pick a growth number so large it's silly — 72 ÷ 72 = 1 year. Then try something tiny: 72 ÷ 1 = 72 years. Seeing both ends shows the shortcut is a relationship, not a magic constant. It also teaches you that a big growth claim should sound suspicious, which is worth more than the arithmetic itself.
Do not turn it into a forecast.
The tempting next move is “so if I saved this much a week until I'm thirty…” Resist it. The moment you attach a growth number to your actual money, you've made a forecast, and it will be wrong. Keep this as a piece of arithmetic you own, nothing more. Not forecasting is the advanced skill hiding in this lesson.
You've got it if you can do 72 ÷ something without looking it up, and if you say “but I made that number up” without anyone prompting you. The second half matters more than the first.
Not financial advice · Educational only