Let's play: Fermi estimates

Fermi estimates are about getting rough guesses from as little data as possible. You win if you get it within a factor of 10.
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.

2. How many fatalities per year from passenger-jet crashes have there been, in the past 20 years?

• A jet crash happens for every 30 years of total flight time. Source: watching too much Air Crash Investigations.
• There are 1e4 hours in a year. Source: memory.
• So, jet crash rate is 1 crash/3e5 hours.
• Every one who can comfortably afford a flight, do fly twice a year. Source: blind guess. It seems a lot of people likes to travel whenever they can, and most travels contain two flights.
• About 1e9 people in the world are earning $\ge\$32/\text{day}$, which is about $\$8000/\text{year}$, definitely comfortable for vacations. Source: I checked online. 
• Each flight ranges from 1 to 10 hours, so on geometric average, a flight lasts 3 hours. Source: common sense. Also, I flew a lot.
• Each jet contains $1e2$ people. Source: common sense.
• So, each year $1e9 / 1e2 * 2 * 3 = 6e7$ hours of flight, giving us... $6e7 \text{ hours}/3e5 \text{ hours per crash} = 200$ crashes per year.
• Each crash kills everyone on board. Source: common sense. Also, I watched too much Air Crash Investigations, and according to that, usually you die in a crash.
• So that gives $200\text{ crashes } * 100\text{ people } = 20000 \text{ people}$

This is worse than I thought.

The correct answer can be seen on Wikipedia:

According to ACRO [Bureau of Aircraft Accidents Archives]... fewer than 170 incidents every year between 2009 and 2017, compared to as many as 226 as recently as 1998... Annual fatalities have been less than 1,000 in nine of the fourteen years since 2004, with 2017 experiencing the lowest number of fatalities, at 399, since the end of World War II.

So my estimate about the incident number is pretty much spot-on, but the number of fatalities is off by 20.

Let's check the numbers online instead of purely relying on my intuition.. According to this, there were 4e9 air passengers in 2009. My estimate of $1e9 * 2$ is pretty much spot-on.

According to FAA, there were 2e7 hours of general aviation in 2015. My estimate of 6e7 hours was also very good, too big by 3.

So it's probably because I way overestimated the number of crashes. According to ICAO, the rate of accidents is 2.8 accidents per million departures. So that means about 3 accidents per 3 million hours, or 1 accident per 1e6 hours. My estimate was now too big by 10.

Well, that's good enough...

3. How much does the New York City government spends on K-12 education every year?

• There are 1e7 people in New York. Source: Memory says that the biggest human cities have about 10 million people.
• Each person lives 70 years, among which 15 years are spent in K-12 education, so that gives 1/5 of life spent in education. 
• So at any point there are $1/5 * 1e7 = 2e6$ people in K-12 education in New York.
• Each class has about 20 people and each grade about 5 classes, so 100 people per grade, or $2e4$ grades. Source: High School Musical.
• Each grade has about 10 teachers, so that's $2e5$ teachers. Source: common sense.
• Each American teacher's salary is about $\$3e4/\text{year}$, so that's $\$6e9/\text{year}$ for New York. Source: memory of reading about job prospects in America.
• The other parts of the education system should also cost more. Let's say each teacher requires a support staff so multiply by 2. Then add in all the equipment costs, so multiply by 2 again.
• So that gives $4 * \$6e9/\text{year} =$ $20 billion/year.

The right answer is, according to this news site, 

the total funding allocated to individual schools would be $23.3 billion [in 2019].

4. How many plays of Only Shallow (1991), by My Bloody Valentine, have been reported to last.fm?

I have no idea what that means... I mean, the song is great (but I actually prefer When You Sleep), but what is last.fm?

Last.fm makes a record of every audio track you play, if you enable the relevant feature or plugin for the music software on your phone, computer, or other device.

Oh, okay. This time I will definitely refer to web searches all the way instead of relying on my memories and intuitions, because I know almost nothing about this.

• last.fm started 16 years ago. Source: search.
• I went to the homepage of the website and in the "Around The World" part, the most trending music today [Steve Aoki ft. BTS - 'Waste It On Me'] in America was played about 3e5 times.
• On Youtube, it was 6e5 times. So the ratio of plays on Youtube vs last.fm is about 2.
• On youtube, Only Shallow has been played about 2 million times in the past 10 years.
• Considering that last.fm became popular only around 2006, I'll just take the past 10 years' estimate. So, my final estimate is 2 million times / 2 = 1 million times.

The correct answer is 3 million according to last.fm.

Well I admit that I did very little thinking for this, but it's just I know so little about normal human music listening, I can't even get started on the problem.