As an academic, I am supposed to criticize Wikipedia as not being a reliable peer-reviewed source of data. In practice, some of the articles are very good. This is one of them:
https://en.wikipedia.org/wiki/Genetic_algorithm
Here is the "do math and calculate a certain number" link. There are three pages and the first page is certainly odd, but it gets better on pages 2 and 3.
http://www.ai-junkie.com/ga/intro/gat1.html
Here is a link to the World with Plants and Eaters Genetic algorithm demo http://math.hws.edu/eck/js/genetic-algorithm/GA.html
The background is linked from that page, but totally worth a call out here: http://math.hws.edu/eck/js/genetic-algorithm/ga-info.html