90

90 is a natural number that comes after 89 and before 91. In decimal notation it is written as 90. In other systems, it is XC in Roman numerals, 1011010 in binary, and 5A in hexadecimal. In ASCII, the code point 90 corresponds to the capital letter Z.

In mathematics, 90 factors as 2 × 3^2 × 5. It has 12 positive divisors. It is an even, composite number. Euler's totient function gives φ(90) = 24. The sum of its proper divisors is 144, making 90 an abundant number. It is also a Harshad number in base 10 since 90 is divisible by the sum of its digits (9).

In geometry, 90 denotes a right angle: an angle of 90 degrees. This is the standard measure for perpendicularity. In radians, 90 degrees equals π/2. In time, 90 minutes equals 1 hour and 30 minutes.

In science, the atomic number 90 corresponds to thorium, a radioactive actinide. Thorium occurs naturally in minerals and has been used historically in gas mantles; thorium-based reactors and fuels have been proposed as alternatives in some nuclear technologies.

In geography and transportation, Interstate 90 is the longest Interstate Highway in the United States, running coast-to-coast from Seattle to Boston. The term 90 is also used in other regional route designs and roadways around the world.

Culturally, a person aged 90 is described as a nonagenarian, a member of the nonagenarian age group.