Here's a trick every kid should know by Class 5. It's one line of math. It protects them from terrible investments for the rest of their lives.
The rule
Years to double your money ≈ 72 ÷ annual interest rate
That's it.
How it works
Say you invest ₹1,000 at 6% a year.
72 ÷ 6 = 12 years.
After 12 years: ~₹2,000.
After 24 years: ~₹4,000.
After 36 years: ~₹8,000.
Now try 12%.
72 ÷ 12 = 6 years.
Same 36 years → ₹1,000 doubles six times → ₹64,000.
Twice the interest rate → 8× the end result. This is why "just 6% more" is never just 6% more. It's everything.
Why kids love it
Because they can DO the math. A 9-year-old can divide 72 by any small number. Give them a number, they tell you how fast money doubles.
Teaching this feels like giving them a superpower. Because, honestly, most adults don't know this rule either.
Try it on your child tonight
- "If your ₹500 pocket money grew at 8% a year, how long before it's ₹1,000?" → 72 ÷ 8 = 9 years
- "And ₹2,000?" → another 9 years (18 total)
- "What if we got 15%?" → 72 ÷ 15 ≈ 5 years per double. That ₹500 becomes ₹1,000 in 5, ₹2,000 in 10, ₹4,000 in 15…
This is how compounding becomes a game, not a formula.