A few days ago
zur2122

# What are some interesting facts about the number 21?

What are some interesting facts about the number 21?

A few days ago
miss lyss

if you flip it.. than it is the number 12

and that is my fav number =]

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4 years ago
?
Number 21 Facts
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A few days ago
Anonymous
Twenty-one is a Fibonacci number, a Harshad number, a Motzkin number, a triangular number and an octagonal number, as well as a composite number, its proper divisors being 1, 3 and 7.

21 is the fifth discrete bi-prime and the second in the (3.q) family. With 22 it forms the second discrete bi-prime pair. 21 has an aliquot sum of 11 though it is the second composite number found in the 11-aliquot tree with. 18 is the first such member as well having as 21 as its aliquot sum 21 is the first number to be the aliquot sum of three numbers 18,51,91

21 appears in the Padovan sequence, preceded by the terms 9, 12, 16 (it is the sum of the first two of these).

The sum of divisors for the integers 1 through 5 is 21.

21 is a repdigit in base 4 (111).

21 is the smallest number of differently sized squares needed to square the square.

It is a proper number.

It is the minimum drinking age in many countries, notably the US.

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5 years ago
Anonymous
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A few days ago
Luke W
1 ) You can get married without parent permission

2 ) Drinking age in most states

3 ) You can gamble

4 ) Cows graze in the wild in groups of an average of 21 cows

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4 years ago
?
1
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A few days ago
embroidery fan
1) Two main factors are prime

2) Age of complete adulthood in the US

3) Counting, backward (instead of 1,2= 12)

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A few days ago
Anonymous
Legal drinking age
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A few days ago
Anonymous
It comes after 20 and before 22….interesting!
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A few days ago
bahaviaa