55 (number)

55 (fifty-five) is the natural number following 54 and preceding 56.

← 54 55 56 →
← 50 51 52 53 54 55 56 57 58 59 → List of numbers Integers ← 0 10 20 30 40 50 60 70 80 90 →
Cardinal	fifty-five
Ordinal	55th (fifty-fifth)
Factorization	5 × 11
Divisors	1, 5, 11, 55
Greek numeral	ΝΕ´
Roman numeral	LV, lv
Binary	1101112
Ternary	20013
Senary	1316
Octal	678
Duodecimal	4712
Hexadecimal	3716

Mathematics

55 is a square pyramidal number.

55 is:

• the 10th Fibonacci number^[1] and the 10th triangular number,^[2] The sum of 55's digits is also 10.
• the 5th heptagonal number,^[3] the 5th square pyramidal number,^[4] and the 4th centered nonagonal number.^[5]

55 is also the 19th semiprime and the 15th squarefree semiprime,^[6][7] as well as the 32nd nontotient number (including odd numbers > 1)^[8] the 36th arithmetic number,^[9] the 38th composite number^[10] (where the abundance of 55 = - 38),^[11] the 43rd deficient number,^[12] the 45th trapezoidal number,^[13] and the 49th polite number.^[14]

In addition, the first 10 non-zero numbers added together equal 55. (1+2+3+4+5+6+7+8+9+10 = 55)

Transportation

• In the United States, the National Maximum Speed Law prohibited speed limits higher than 55 miles per hour (90 km/h) from 1974 to 1987. The number 55 became a popular shorthand for the 55 mph speed limit. For example, a hand with a pair of fives in Texas hold'em poker is referred to as a "speed limit".^[15]