400 (number)
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400 (four hundred) is a natural number that comes after 399 and before 401. It is a whole number and can be written as 4 × 100.
In the decimal number system, 400 consists of three digits: 4, 0, and 0. It belongs to the whole numbers commonly used in measurements, history, and geometry.
Mathematical properties
400 is a perfect square because 20 × 20 = 400. Its square root is therefore 20, a whole number. It is a composite number, not a prime number.
Its prime factorization is 2⁴ × 5². Its divisors include 1, 2, 4, 5, 8, 10, 16, 20, 25, 40, 50, 80, 100, 200, and 400 itself.
Geometry
In geometry, the number 400 can represent the area of a square with sides of 20 units. Additionally, in the unit system called the grad, a complete rotation is divided into 400 parts, whereas the conventional system divides it into 360 degrees.
Other uses
In history, 400 can refer to the year 400 AD or 400 BC. In the Gregorian calendar, a century year is a leap year only if it is divisible by 400; for example, 1600 and 2000 were leap years, while 1700, 1800, and 1900 were not.