Charlene wants to invest $10,000 long enough for it to grow to at least $20000. The compound interes

Charlene wants to invest $10,000 long enough for it to grow to at least $20,000. The compound interest rate is 6% p.a. How many whole number of years does she need to invest the money for so that it grows to her $20,000 target?

We want 10,000(1.06)^n = 20,000.

But what the problem asks for is how long it will take money to double. We can use a shortcut called the Rule of 72. Using the Rule of 72 at 6%, we get 12 years.