ninety

Ninety is the natural number that comes after eighty-nine and before ninety-one. In the decimal system it is written as 90, and in Roman numerals it is XC. It is an even composite number, used to denote a count of ninety units in various contexts.

Etymology: The word ninety comes from nine plus the suffix -ty, from Old English tīġ meaning ten or tenfold. Through regular sound changes in Middle English the form evolved into ninety.

Mathematics and representation: 90 = 2 × 3^2 × 5. Its divisors include 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, and 90. It is a Harshad (Niven) number in base 10 since 90 is divisible by the sum of its digits (9 + 0 = 9). In binary, 90 is 1011010.

Uses and occurrences: In geometry, a 90-degree angle is a right angle. In timekeeping and sports, many games are described as lasting 90 minutes. In science, 90 is the atomic number of thorium, a radioactive element that has been used in gas mantles and in certain high-temperature alloys. The number also appears in various numerical and measurement contexts as a common milestone, such as in angles, temperatures, and ratings.

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