Answer:
8.0
Step-by-step explanation:
The attached shows guesses until we have an answer accurate to 6 decimal places. Rounded to 1 decimal place, the answer is 8.0 years.
This (8 years) was our initial guess, based on the "rule of 72" which says the doubling time for 9% interest is 72/9 = 8 years.
Our second guess, at 8.1, was farther away from the doubled value, so the first guess at 8.0 is closer.
t = 8.0
