the consumer price index is increasing at a rate of 9% per year. what is its doubling time?

Answer 1

8 years, without using a calculator!

The rule of 72 gives doubling time as the quotient of 72 over the interest rate.

An accurate calculation is as follows:
n=number of years:
1.09^n=2
take logs and solve for n:
n=log(2)/log(1.09)
=8.04 years