mathematics

20 (number)

20 (number) is a mathematics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand 20 (number) rather than just read about it. In short: 20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units is sometimes referred to as a score.

Key takeaways

20 (number) belongs to mathematics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect 20 (number) to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of 20 (number) from memory before moving on to harder problems.

Reference excerpt

20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units is sometimes referred to as a score.

In mathematics Twenty is a composite number. It is also the smallest primitive abundant number because all of its proper divisors are deficient. The Happy Family of sporadic groups is made up of twenty finite simple groups that are all subquotients of the friendly giant, the largest of twenty-six sporadic groups.

Geometry An icosagon is a polygon with 20 edges. Bring's curve is a Riemann surface, whose fundamental polygon is a regular hyperbolic icosagon.

Platonic solids

The largest number of faces a Platonic solid can have is twenty faces, which make up a regular icosahedron. A dodecahedron, on the other hand, has twenty vertices, likewise the most a regular polyhedron can have. This is because the icosahedron and dodecahedron are duals of each other.

Other fields

Science

Physics 20 is the third magic number in physics.

Biology In some countries, the number 20 is used as an index in measuring visual acuity. 20/20 indicates normal vision at 20 feet (6.1 m), although it is commonly used to mean "perfect vision" in 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".

Psychology In many disciplines of developmental psychology, adulthood starts at age 20.

Culture

Age 20 The traditional age of majority in Japan, although the voting age has been reduced to 18. Japanese people commemorate the twentieth birthday with personal ceremonies, and it comes with a number of legal rights like the right to marry. To represent this, the Japanese language has a special word for "20-years-old" that does not follow the rest of their numbering system. Accordingly, the word 二十歳 is read all at once as "はたち" (hatachi) rather than the expected pronunciation of the three characters as "にじゅうさい" (nijyuusai, which is literally "two," "ten," and the counter for "years old").

Number systems 20 is the basis for vigesimal number systems, used by several different civilizations in the past (and to this day), including the Maya.

Indefinite number A 'score' is a group of twenty (often used in combination with a cardinal number, e.g. fourscore to mean 80), but also often used as a large, indefinite number (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila").

References

External links

Media related to 20 (number) at Wikimedia Commons

Worked examples

Example 1 — a first encounter with 20 (number)

Start with the simplest possible case. Write down what 20 (number) claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In mathematics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to 20 (number) before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about 20 (number) ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of 20 (number)

In research: 20 (number) appears in mathematics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses 20 (number) in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: 20 (number) is common in secondary-school and first-year university syllabi. It links to neighbouring topics 20 (number), Integers, so understanding it makes those chapters shorter.
In everyday life: Look for 20 (number) outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “20 (number)” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study 20 (number) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what 20 (number) means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain 20 (number) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is 20 (number) in simple terms?

20 (twenty) is the natural number following 19 and preceding 21. A group of twenty units is sometimes referred to as a score.

Why does 20 (number) matter?

Because it connects several mathematics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study 20 (number)?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on 20 (number).