At current growth rates, the Earth's population is doubling about every 69 years.

At current growth rates, the Earth's population is doubling about every 69 years. If this growth rate were to continue, about how many years will it take for the Earth's population to become one-fifth larger than the current level?

I am stuck because I don't think the years can be a negative number. This is what I did P_{o}2^(t/69)=1/5P_{o}

Re: At current growth rates, the Earth's population is doubling about every 69 years.

$\displaystyle \frac{6}{5} \cdot P = P \cdot 2^{\frac{t}{69}}$

solve for t ...