The rule of 70 states that to estimate the number of years required to double your income (or investment) at a certain annual growth rate, you can divide 70 by the growth rate percentage.
In this case, if your income grows at 10 percent per year, you would calculate it as follows:
\[ \text{Number of years to double} \approx \frac{70}{\text{growth rate}} = \frac{70}{10} = 7 \text{ years} \]
Thus, the answer is:
a. 7 years.