Number reference
85 explained.
85: eighty-five, composite; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- eighty-five
- Classification
- composite
- Factors
- 1, 5, 17, 85
- Prime factorization
- 5 × 17
- Square root
- 9.219544
- Cube root
- 4.39683
- Binary
- 1010101
- Octal
- 125
- Hexadecimal
- 55
- Roman numeral
- LXXXV
- Digit sum
- 13
- Digital root
- 4
What makes 85 notable
- Semiprime. 85 = 5 × 17, a product of exactly two primes. That gives it just four divisors, so it splits into equal groups in only two non-trivial ways.
85 in everyday use
Eighty-five is the atomic number of astatine, the rarest naturally occurring element: less than a gram exists in the Earth's crust at any moment, and it has never been seen in bulk. In the United States, 85 miles per hour is the highest posted speed limit, on a stretch of Texas toll road. E85 fuel takes its name from containing up to 85 percent ethanol.
Eighty-five is 5 × 17 and the third number, after 50 and 65, that is a sum of two squares in two ways: 4 + 81 and 36 + 49. In binary it is 1010101, alternating ones and zeros, and in hexadecimal it is 55, a repdigit. Because both its prime factors leave a remainder of 1 when divided by 4, it is the hypotenuse of two different right triangles with whole-number sides.
Nearby numbers
How to read this page
The factor list shows every way 85 splits into equal whole-number groups — 4 divisors in total. The prime factorisation is the compact form of that same information, and the base conversions restate 85 for contexts that do not use decimal.
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