How many events make up an Olympic decathlon?

How to get close to this number

The name is the answer, and noticing that is the whole trick. "Deca" is the Greek prefix for ten, the same root as decade and decagon. A surprising number of questions about sports, geometry and calendars are answered by reading the word rather than recalling the fact, and this is the cleanest example on the site.

The explanation extends the trick to its sibling event: the heptathlon uses "hepta" for seven. Once you have the pattern, an entire family of questions becomes trivial — pentathlon, triathlon, biathlon all name their own answers. Etymology is an underrated estimation tool, especially in sport, where classical naming survived.

What the name does not tell you is the structure: ten events spread across two days, combining sprints, jumps, throws and a distance race, scored by a points table rather than by placing. The scoring formula is what makes the event coherent, and it has been revised several times — so the event count is fixed while what it takes to win is not.