# 90 (number)

 ← 89 90 91 →
Cardinal ninety
Ordinal 90th
(ninetieth)
Factorization 2 × 32× 5
Divisors 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90
Roman numeral XC
Binary 10110102
Ternary 101003
Quaternary 11224
Quinary 3305
Senary 2306
Octal 1328
Duodecimal 7612
Vigesimal 4A20
Base 36 2I36

90 (ninety) is the natural number preceded by 89 and followed by 91.

In English speech, the numbers 90 and 19 are often confused, as sounding very similar. When carefully enunciated, they differ in which syllable is stressed: 19 /naɪnˈtiːn/ vs 90 /ˈnaɪnti/. However, in dates such as 1999, and when contrasting numbers in the teens and when counting, such as 17, 18, 19, the stress shifts to the first syllable: 19 /ˈnaɪntiːn/.

Interstate 90 is a freeway that runs from Washington to Massachusetts.

## In mathematics

• Because 90 is the sum of its unitary divisors (excluding itself), it is a unitary perfect number, and because it is equal to the sum of a subset of its divisors, it is a semiperfect number. 90 is a pronic number. But it is also a nontotient. It is a Perrin number, preceded in the sequence by 39, 51, 68.
• In normal space, the interior angles of a square measure 90 degrees each. Also, in a right triangle, the angle opposing the hypotenuse measures 90 degrees, with the other two angles adding up to 90 for a total of 180 degrees.[1] Thus, an angle measuring 90 degrees is called a right angle.[2]
• 90 is divisible by the sum of its base 10 digits, thus it is a Harshad number.