The number 28
28 is a perfect number: its divisors below itself add up to 28 and a triangular number: 1 + 2 + … + 7 = 28. Its prime factorisation is × 7. Divisors (6): 1, 2, 4, 7, 14, 28.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 28: a perfect number: its divisors below itself add up to 28; a triangular number: 1 + 2 + … + 7 = 28; a happy number: repeatedly adding the squares of its digits reaches 1.
- 2
28 is composite. Its prime factorisation is
- 3
The divisors add up to 56; without 28 itself that is 28, so 28 is a perfect number. Euler's totient φ(28) = 12 numbers up to 28 share no factor with it.
- 4
The digits of 28 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 18 steps to reach 1.
LaTeX source of the formulas on this page
\[ 28 = 2^{2} \times 7 \]28 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 6 |
| Sum of divisors | 56 |
| Euler totient φ(n) | 12 |
| Square | 784 |
| Cube | 21952 |
| Square root | 5.291503 (= 2√7) |
| Digit sum / digital root | 10 / 1 |
| Collatz steps to 1 | 18 |
| Scientific notation | 2.8 × |
28 in other forms
| Form | Value |
|---|---|
| In words | twenty-eight |
| Roman numerals | XXVIII |
| Binary (base 2) | 11100 |
| Octal (base 8) | 34 |
| Hexadecimal (base 16) | 1C |
| Base 3 | 1001 |
| As time | 28 seconds |
Is 28 divisible by 2 to 12?
| Divisor | Answer | Rule |
|---|---|---|
| 2 | Yes (28 ÷ 2 = 14) | 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 | Yes (28 ÷ 4 = 7) | A number is divisible by 4 if the last two digits make a multiple of 4. |
| 5 | No (remainder 3) | A number is divisible by 5 if the last digit is 0 or 5. |
| 6 | No (remainder 4) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | Yes (28 ÷ 7 = 4) | 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 4) | 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 8) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 6) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 4) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 28 a prime number?
No. 28 = × 7, so it is divisible by 2.
What are the factors of 28?
1, 2, 4, 7, 14, 28.
What is 28 in binary?
11100 (hexadecimal 1C, octal 34).
What is the square root of 28?
About 5.2915, exactly 2√7.
What is 28 in Roman numerals?
XXVIII.
Is 28 divisible by 3?
No. Its digits add up to 10, which is not a multiple of 3.
What is special about the number 28?
28 is a perfect number: its divisors below itself add up to 28; a triangular number: 1 + 2 + … + 7 = 28; a happy number: repeatedly adding the squares of its digits reaches 1.