3QUESTION 3 OF 3
Answer & explanation
The official answer, with Fermi’s reasoning step by step.
Answer & explanation
Official answer
2,000,000,000
people
How to estimate it
Fermi’s official “Napkin math”
Start with today's world population, about 8 billion. How much smaller was it a century ago? Maybe a quarter of today's number.
- World population today
- ≈ 8,000,000,000
- Guessed share alive a century ago
- ≈ 1 in 4
- 8,000,000,000 ÷ 4
- ≈ 2,000,000,000
Rounded assumptions may give a slightly different estimate. The official answer above is the game’s reference value.
Source calculation & reference
- World population, 1920 (U.S. Census Bureau historical estimate)
- 1,860,000,000
- World population, 1930 (U.S. Census Bureau historical estimate)
- 2,070,000,000
- 1925 midpoint: (1,860,000,000 + 2,070,000,000) ÷ 2
- 1,965,000,000
- Rounded estimate
- ≈ 2,000,000,000
Reference cited by Fermi: U.S. Census Bureau historical estimates, interpolated between 1920 and 1930
How close was your guess?
Your comparison will appear here.
Compare by ratio: divide the larger number by the smaller one. The closer to 1×, the better.