3QUESTION 3 OF 3
Answer & explanation
The official answer, with Fermi’s reasoning step by step.
Answer & explanation
Official answer
60,000,000
people
How to estimate it
Fermi’s official “Napkin math”
The easiest place to start is the world's population, about 8 billion. How many of them are identical twins? It's rare, you might know one pair. Maybe 1 in 100 people.
- World population
- 8,000,000,000
- Identical twins
- ≈ 1 in 100
- 8B ÷ 100
- ≈ 80,000,000
Rounded assumptions may give a slightly different estimate. The official answer above is the game’s reference value.
Source calculation & reference
- Identical twin pairs per 1,000 deliveries, about the same everywhere (Monden, Pison and Smits, Human Reproduction 2021)
- ≈ 4
- Two babies per pair, so share of people born an identical twin
- ≈ 0.8%
- World population 8.2B × 0.8%
- ≈ 65,000,000
- Less twins' higher child mortality, rounded
- ≈ 60,000,000
Reference cited by Fermi: Monden, Pison and Smits, Human Reproduction (2021): about 4 identical pairs per 1,000 deliveries
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.