Simplify the following text for a Grade 8 native English-speaking person.
Original: Therefore , GDP can be stated as GDP per capita ( per person ) in which total GDP is divided by the resident population on a given date , GDP per citizen where total GDP is divided by the numbers of citizens residing in the country on a given date , and less commonly GDP per unit of a resource input , such as GDP per GJ of energy or Gross domestic product per barrel .
Simplified: In economics , the gross domestic product ( GDP ) is how much a place produces in some amount of time . For nations , the GDP can be found by adding up its output inside the borders of that country .