Definitions

# 20 (number)

"Twenty" redirects here. For the village in England, see Twenty, Lincolnshire.
20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units may also be referred to as a score.

## In mathematics

Twenty is a composite number, its proper divisors being 1, 2, 4, 5 and 10. Skipping 2 and adding the other divisors gives 20, hence 20 is a semiperfect number, and since it is not divisible by any of the smaller semiperfect numbers, it is a primitive semiperfect number. 20 is the smallest abundant number that is not a multiple of 3.20 is also a composite number.

20 is the second positive integer with a prime factorization of the form $2^2 cdot q,$ where q is a prime. Twenty is the third number in abundance (110%) with the aliquot sum of 22 within the 8-member aliquot sequence (20,22,14,10,8,7,1,0). Twenty is the aliquot sum of the discrete biprime, 34 and the square 361.

It is also the sum of the first four triangular numbers, making it a tetrahedral number.

20 is the total number of digits on the human body, which makes it a convenient number system radix; such number systems are called vigesimal. In the former British currency system, there were twenty shillings in a pound.

A polyhedron with twenty faces is an icosahedron.

Twenty is the first number in an early Indian magic square of order 3.

In base 10, it is a Harshad number and the smallest even self number.

The Monster group contains 20 sporadic groups.

## In science

### Biology

• The number of proteinogenic amino acids that are encoded by the standard genetic code.
• In some countries, the number 20 is used as an index in measuring visual acuity. 20/20 indicates normal vision at 20 feet, although it is commonly used to mean "perfect vision" (Note that this applies only to countries using the Imperial system. The metric equivalent is 6/6). When someone is able to see only after an event how things turned out, that person is often said to have had "20/20 hindsight".
• There are 20 baby teeth in the deciduous dentition.

20 is:

## Historical years

20 A.D., 20 B.C., 1920, 2020, etc.

## References

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