Challenge problem · non-routine

Answer: 11 years

1. We need $1.07^n \ge 2$.
2. $1.07^{10} \approx 1.967 < 2$ and $1.07^{11} \approx 2.105 \ge 2$.
3. So it takes 11 whole years. (Using logarithms, $n = \log 2 / \log 1.07 \approx 10.24$, rounded up.)