To solve the curious question, Marshall and his students applied contemporary models used to determine animal populations and offered some educated guesses regarding the dinosaur's physiology and the range within North America that the creature probably lived. After plugging these possibilities into the equation, they found that the probable population of adult T. Rexes in North America at any given time would have been around 20,000. Since the dinosaurs are believed to have existed for around 2.4 million years, this would mean that approximately 2.5 billion T. Rexes once roamed the Earth.