25 (number)
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25 is a natural number that comes after 24 and before 26. It is written as 25 in the Hindu-Arabic numeral system and XXV in Roman numerals. It is a composite number, being the product of 5 multiplied by 5.
Because 25 is a perfect square, its divisors are 1, 5, and 25. It therefore belongs to the class of numbers with exactly three divisors, which means its square root, 5, is a prime number.
Mathematical properties
25 is the fifth perfect square. It is also one of the three most famous Pythagorean triples: 3² + 4² = 9 + 16 = 25, meaning that 25 can be expressed as the sum of two perfect squares. The sum of its proper divisors (1 and 5) is 6, which is less than 25, making it a deficient number.
When 25 is multiplied by other numbers ending in 5, the result always ends in either 25 or 75, arising from the fact that 25 is an automorphic number: 25² = 625, which ends in 25.
Numeral systems
In binary, 25 is 11001. In hexadecimal it is 19. Because 100 is divisible by 25 four times, 25 appears regularly in percentage calculations: 25% is one quarter (¼).
Common uses
Many monetary systems have a denomination of 25, such as the 25-cent coin in some countries. The 25th wedding anniversary is commonly called a "silver jubilee" in Western languages. 25 is also used in the division of time and various measurements, particularly where a quarter (¼) is indicated.