The number 55
55 is a Fibonacci number, a triangular number: 1 + 2 + … + 10 = 55 and a palindrome: it reads 55 backwards too. Its prime factorisation is 5 × 11. Divisors (4): 1, 5, 11, 55.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 55: a Fibonacci number; a triangular number: 1 + 2 + … + 10 = 55; a palindrome: it reads 55 backwards too; a semiprime: 5 × 11, the product of two primes.
- 2
55 is composite. Its prime factorisation is
- 3
The divisors add up to 72; without 55 itself that is 17, so 55 is deficient (17 is less than 55). Euler's totient φ(55) = 40 numbers up to 55 share no factor with it.
- 4
The digits of 55 add up to 10, and repeating that until one digit is left gives the digital root 1. The Collatz sequence (halve if even, else 3n+1) takes 112 steps to reach 1.
LaTeX source of the formulas on this page
\[ 55 = 5 \times 11 \]
55 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 4 |
| Sum of divisors | 72 |
| Euler totient φ(n) | 40 |
| Square | 3025 |
| Cube | 166375 |
| Square root | 7.416198 |
| Digit sum / digital root | 10 / 1 |
| Collatz steps to 1 | 112 |
| Scientific notation | 5.5 × |
55 in other forms
| Form | Value |
|---|---|
| In words | fifty-five |
| Roman numerals | LV |
| Binary (base 2) | 110111 |
| Octal (base 8) | 67 |
| Hexadecimal (base 16) | 37 |
| Base 3 | 2001 |
| As time | 55 seconds |
Is 55 divisible by 2 to 12?
| Divisor | Answer | Rule |
|---|---|---|
| 2 | No (remainder 1) | A number is divisible by 2 if the last digit is even (0, 2, 4, 6, 8). |
| 3 | No (remainder 1) | A number is divisible by 3 if the digits add up to a multiple of 3. |
| 4 | No (remainder 3) | A number is divisible by 4 if the last two digits make a multiple of 4. |
| 5 | Yes (55 ÷ 5 = 11) | A number is divisible by 5 if the last digit is 0 or 5. |
| 6 | No (remainder 1) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 6) | A number is divisible by 7 if double the last digit and subtract it from the rest: the result is a multiple of 7. |
| 8 | No (remainder 7) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 1) | A number is divisible by 9 if the digits add up to a multiple of 9. |
| 10 | No (remainder 5) | A number is divisible by 10 if the last digit is 0. |
| 11 | Yes (55 ÷ 11 = 5) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 7) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 55 a prime number?
No. 55 = 5 × 11, so it is divisible by 5.
What are the factors of 55?
1, 5, 11, 55.
What is 55 in binary?
110111 (hexadecimal 37, octal 67).
What is the square root of 55?
About 7.4162.
What is 55 in Roman numerals?
LV.
Is 55 divisible by 3?
No. Its digits add up to 10, which is not a multiple of 3.
What is special about the number 55?
55 is a Fibonacci number; a triangular number: 1 + 2 + … + 10 = 55; a palindrome: it reads 55 backwards too; a semiprime: 5 × 11, the product of two primes.