A mathematical approximation called the rule of 70 tells us how long it will take for something to double in size if it grows at a constant rate. The doubling time is approximately equal to the number 70 divided by the percentage rate of growth. Thus, if Panama's real GDP per person is growing at 7 percent per year, it will take about 10 years (= 70/7) to double. Apply the rule of 70 to solve the following problem:

Suppose that real GDP per person in Panama in2017was about $15,000per person, while it was about $60,000per person in the United States. If real GDP per person in Panama grows at the rate of 4 percent per year, about how long will it take Panama's real GDP per person to reach the level that the United States was at in 2017? (Hint: How many times would Panama's 2017 real GDP per person have to double to reach the United States' 2017 real GDP per person?)