A useful rule of thumb called the "rule of 70" states that if something grows at a constant rate of z percent per year, it doubles in size approximately every __________ years.
a. z/70
b. 70/z
c. 70 - z
d. 70 × (z/100)
The answer is b. (70/z). If I put $100 into a savings account and the annual growth rate is 10%, the 70/10%= 7 years that the initial $100 would take to double.
