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Chapter 4
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The average life expectancy is nearly 80 for women, and somewhat lower for men. A good first guess of the number of people in each year of life, then, is just the total population divided by the number of years of life (on average):
3 x 108 / 75 = 4,000,000. This isn't exact, of course; there is a huge bulge from those born in the "baby boom" years of 1946-1956 and an echo of that boom roughly 25 years later. But it is a good first approximation (see Chapter 5 on the use of approximations in science.)