The “Rule of 70,” which is to divide 70 by the rate of growth, gives us the time it takes for a country to double its output.
Years to double = (70 / Rate of Growth)
Since the rate of growth for real GDP per capita is 5% in the follower country, the country's real GDP per capita will double every 14 years.
Years to double = (70 / 5)=14
So, in 14 years the follower country's real GDP per capita will be $40,000 given its current level of $20,000.
The rate of growth for real GDP per capita in the leader country is 0% (no growth).
Thus,it will remain at $40,000 real GDP per capita. Combining the two results above, it will take 14 years for the follower country to catch-up with the leader country.