Fermi Problems

A Fermi problem asks you to estimate something with no obvious data. How many golf balls fit in a school bus? How many gas stations operate in your country? You start with what you know — population, geography, rough usage rates — and chain estimates together logically.

The trick is bounding your answer. You know the answer is more than X and less than Y. Then narrow the range with each additional fact. Professionals — scientists, engineers, investors — use this technique daily to make fast, reasonable decisions before detailed data arrives.