The central thesis of Smith's "The Wealth of Nations" is that our need to fulfill self-interest results in prosperity. Smith believed that people promote public interest through economic choices—a free-market force that became known as the "invisible hand."
Well to find this out you need to make an equation
s+7=j
So for example if Johnny's sister is 15 then Johnny would be 22 because
s(sister's age) +7(number of years difference between their ages)=22(Johnny's age)
