And that's slightly longer (buy not by much) than the time it took to double from 2 to 4 billion (1927-1974), and quite a bit longer than the time taken from 3

And that's slightly longer (buy not by much) than the time it took to double from 2 to 4 billion (1927-1974), and quite a bit longer than the time taken from 3 to 6 billion (1960-1999).