Final answer:
The difference between a 3% and 4% rate of growth in the number of years it would take to double the United States' economy is about 23 years.
correct option is a. about 23 years.
Step-by-step explanation:
The difference between a 3% and 4% rate of growth in the number of years it would take to double the United States' economy can be calculated using the rule of 70.
The rule of 70 states that you can find the approximate number of years it takes for a quantity to double by dividing 70 by the growth rate in percent. In this case, dividing 70 by 3 gives us approximately 23 years, and dividing 70 by 4 gives us approximately 17.5 years. Therefore, the answer is about 23 years.