85 (Number)
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85 is a natural number that comes after 84 and before 86. It consists of two digits (eight tens and one unit) and is written in the standard base-ten system used globally.
Like other small numbers, 85 is used for counting, measurement, reference, and classification. It has mathematical properties that arise from its divisibility.
Mathematical Properties
85 is a composite number, meaning it has divisors other than 1 and itself. It equals 5 × 17. Its complete set of divisors is 1, 5, 17, and 85. Both 5 and 17 are prime numbers, so 85 is the product of two distinct primes—a type sometimes called a semiprime.
85 is an odd number, since it is not divisible by 2. The sum of its proper divisors (1 + 5 + 17) is 23, which is less than 85; therefore it is a deficient number. It can also be expressed as the sum of two squares in two distinct ways: 85 = 2² + 9² = 6² + 7².
Notation and Other Systems
In Roman numerals, 85 is written LXXXV. In binary notation it is 1010101, an alternating pattern of zero and one—for this reason 85 is often used in computer and electronics testing. In hexadecimal notation it is 55.
General Use
85 appears in everyday numbering systems: a person's age, a percentage (85%), road and vehicle registration numbers, and administrative classification in some countries. The significance of the number depends on the system in which it is used and is not an inherent property of the number itself.