Answer:
23,33 years.
Step-by-step explanation:
We can use the rule of 70 to calculate what the value of a variable might be in the future(taking into consideration that it is an estimate). This rule is pretty efective to determine the amount of years that take a variable to double.
The rule of 70 is representated in this formula:

The annual rate of Growth is 3%
We replace:


Note: In the formula we need to put the percentage not divided by 100 but complete, so we put 3 instead of 0.03 for the formula to work.
At a constant growing rate of 3% per year, the United States would rake 23,33 years to double its real GDP per capita.