Prolog-basic-example/3.arithmetic-density.pl at master · firstneverrest/Prolog-basic-example · GitHub

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% calculate density of the population

population(china, 200).
population(india, 180).
population(japan, 80).
population(thailand, 30).

area(china, 10).
area(india, 5).
area(japan, 2).
area(thailand, 3).

density(X,Y):-population(X,Pop),
              area(X, Ar),
              Y is Pop/Ar.