- 20 is an even composite number.
- 20 is the 4th tetrahedral number.
- 20 is the smallest number with abundance 2.
- The number eπ − π is almost equal to 20. The emphasis is on “almost”, since π and eπ are algebraically independent.
- In Greek-based number-naming systems, 20 is associated with prefix "icosa-", and with prefix "viginti-" in Latin systems.
- 20 is the base of a googologism by Sbiis Saibian called menger sponge. It is equal to 20^3###^^^###3 in Extended Cascading-E Notation.
- Wiki user Denis Maksudov coined the name unalum for this number, and it's equal to \(f_1(10)\) using the fast-growing hierarchy.
- Nirvana Supermind coined the name zero-binol for this number, and it's equal to Q<10,2> using quick array notation. 
As a cash denomination
In pope numbering
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