I took the Fermi estimate problems from this site. You can read how it's done over there. I'll just record my own estimates here.
1. How many new passenger cars are sold each year in the USA?
- There are 3e8 people in America. Source: my memory.
- A citizen of a developed country lives 1e2 years. Source: common knowledge.
- About 1/2 Americans are rich enough to own cars. Source: A feely guess.
- Each American car owner goes through 3 cars in their lives. Source: blind intuition. After the facts, I can justify by saying a car usually breaks down in 20 years, and a person's adult life lasts about 60 years.
So in total, we get
$$(3e8 \text{ person}) * 1/2 * (3 \text{ cars/person }) * / (1e2 \text{ year }) = 4.5e6 \text{ cars/year}$$
The right answer is, according to Wikipedia:
In the year 2009, about $5.5$ million new passenger cars were sold in the United States according to the U.S. Department of Transportation.
