## Fermi Questions Marathon

samm547
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### Re: Fermi Questions Marathon

Schrodingerscat wrote: If cars were to run on the ink out of printer cartridges with the same "ink efficiency" as gas efficiency, how much would you spend on ink to drive from Dallas, TX to Chicago, IL?
Assumptions: Car gets 20mpg, Dallas to Chicago is 2E3 miles, Ink cartrige is 20 ml and costs 25\$, 2L is 1 gallon

Then it is simple multiplication and division, giving 2E3/20*2*1000/20*25 = 2.5E5

What is the 1,000,000th prime number? (This was on my regional test) If stuck, use prime number theorem.

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### Re: Fermi Questions Marathon

samm547 wrote:
Schrodingerscat wrote: If cars were to run on the ink out of printer cartridges with the same "ink efficiency" as gas efficiency, how much would you spend on ink to drive from Dallas, TX to Chicago, IL?
Assumptions: Car gets 20mpg, Dallas to Chicago is 2E3 miles, Ink cartrige is 20 ml and costs 25\$, 2L is 1 gallon

Then it is simple multiplication and division, giving 2E3/20*2*1000/20*25 = 2.5E5

What is the 1,000,000th prime number? (This was on my regional test) If stuck, use prime number theorem.
Okay, this is going to be a stream-of-consciousness problem.
good god, this sounds difficult. not really sure how to approach this mathematically. Well I remember that in the first 100 numbers, 25 or so are prime. so that's about 1/4th, right? but i feel like as you go up it'll get lower, like maybe 1/8th or so. Let's say around 1/8th then. So one million primes, wow that's a lot of primes. holy cow that's huge hehe. uhm so really going by that ratio by the 8 millionth number we should have about the lucky number somewhere by then. So I'd say 7? [/stream]

What is the density of the universe (on average) in kg/cubic light year?
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Schrodingerscat
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### Re: Fermi Questions Marathon

I believe there are a billion galaxies averaging 1 trillion stars each, so will go with 10^21 stars, each weighing about 10^30 kg, so 10^51 kg.
The universe has a radius of I believe around 5*10^10 lys, so 100*10^30 or 10^33 cubic light years.
so thus the density is 10^18 kg/ly^3. Fermi answer 18.

What is the cost of a kilogram of Silver nitrate?

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### Re: Fermi Questions Marathon

Schrodingerscat wrote: What is the cost of a kilogram of Silver nitrate?
Ok, so silver nitrate itself is a bit expensive. I flipped through Ward's a few weeks ago and I remember that like 25 grams was about E2 dollars or so. I think. So multiply by 4E1 and you get E3 dollars (whoa, expensive!)

How many trees would it take to remove all of the CO2 produced by the US in one year?
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Phenylethylamine
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### Re: Fermi Questions Marathon

samm547 wrote:
Schrodingerscat wrote: If cars were to run on the ink out of printer cartridges with the same "ink efficiency" as gas efficiency, how much would you spend on ink to drive from Dallas, TX to Chicago, IL?
Assumptions: Car gets 20mpg, Dallas to Chicago is 2E3 miles, Ink cartrige is 20 ml and costs 25\$, 2L is 1 gallon

Then it is simple multiplication and division, giving 2E3/20*2*1000/20*25 = 2.5E5
Just for the record, a gallon is actually about 4L, so your answer should be 5E5, and therefore 6 rather than 5.
Schrodingerscat wrote:What is the cost of a kilogram of Silver nitrate?
Molecular weight of silver is about 100, nitrate is 14 + 3(16) = 60ish (both slightly low), so silver nitrate is 5/8 silver by mass; let's say the cost of silver nitrate is determined entirely by the cost of the silver it contains, so you have 5/8 kilo of silver, or 6.25E-1 kg; silver is roughly \$30/oz, one ounce is 28g or 2.8E-2 kg:

6.25E-1/2.8E-2 = 2.25E1; 2.25E1*3E1 = 7E2; so 3.

Edit: oops, I was too slow; someone else already answered...
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samm547
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### Re: Fermi Questions Marathon

In response to the 1,000,000th prime number, the mathematical answer uses the prime number theorem: The number of primes less than n asymptotically approaches n/ln(n), which can be manipulated to show that the nth prime number is approximately n*ln(n) or 1E6*ln(1E6) = 6E6*ln(10)~ 3E7, so 7.

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### Re: Fermi Questions Marathon

hers one from our reginals: a pancake is baked that is big enough to cover the surface of the earth. a)how many eggs are required? b) this pancake is shared equally by the population of Michigan, how many calories does each person consume? C) how much weight (in pounds) is gained by each person?
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### Re: Fermi Questions Marathon

so the surface area of the earth is E14 square meters, each single egg will probably produce a pancake that is a circle with a radius around 10 cm, so 3E-2 square meters, so you will need 3E16 eggs so 16. Then the population of Michigan is probably around 10 million, definitely E7. Each standard sized pancake, of which you will make E16, will contain around 1E2 calories so E18 calories over E7 people, E11 calories each. So each pancake probably weighs around 1E2 g, so there are 3E18 g of pancakes to eat, so each person consumes 3E11 of these grams, or 3E8 kg or 6E8 lb which would make the answer 9.

What fraction of the earth's landmass is in [insert your state here]?
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A123456789
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### Re: Fermi Questions Marathon

I live in Michigan.
Michigan is probably about 3% of the United States's landmass, and the United States is probably around 5%-8% of the world's landmass, so the answer is -3.

What fraction of a human's mass is DNA?

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### Re: Fermi Questions Marathon

A123456789 wrote:I live in Michigan.
Michigan is probably about 3% of the United States's landmass, and the United States is probably around 5%-8% of the world's landmass, so the answer is -3.

What fraction of a human's mass is DNA?
A human is about 70 kg, and we have... tens of grams? hundreds of grams? worth of DNA, so -3.

How long (days) would it take to bicycle from New York to San Francisco via Chicago, assuming you're biking only during the day?
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