The number 640
640 is a composite number with 16 divisors. Its prime factorisation is × 5. Divisors (16): 1, 2, 4, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 320, 640.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 640: a Harshad number: divisible by its digit sum 10.
- 2
640 is composite. Its prime factorisation is
- 3
The divisors add up to 1530; without 640 itself that is 890, so 640 is abundant (890 is more than 640). Euler's totient φ(640) = 256 numbers up to 640 share no factor with it.
- 4
The digits of 640 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 12 steps to reach 1.
LaTeX source of the formulas on this page
\[ 640 = 2^{7} \times 5 \]640 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 16 |
| Sum of divisors | 1530 |
| Euler totient φ(n) | 256 |
| Square | 409600 |
| Cube | 262144000 |
| Square root | 25.298221 (= 8√10) |
| Digit sum / digital root | 10 / 1 |
| Collatz steps to 1 | 12 |
| Scientific notation | 6.4 × |
640 in other forms
| Form | Value |
|---|---|
| In words | six hundred and forty |
| Roman numerals | DCXL |
| Binary (base 2) | 1010000000 |
| Octal (base 8) | 1200 |
| Hexadecimal (base 16) | 280 |
| Base 3 | 212201 |
| As time | 640 seconds = 10 min 40 s |
Is 640 divisible by 2 to 12?
| Divisor | Answer | Rule |
|---|---|---|
| 2 | Yes (640 ÷ 2 = 320) | 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 (640 ÷ 4 = 160) | A number is divisible by 4 if the last two digits make a multiple of 4. |
| 5 | Yes (640 ÷ 5 = 128) | 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 | No (remainder 3) | 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 | Yes (640 ÷ 8 = 80) | 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 | Yes (640 ÷ 10 = 64) | A number is divisible by 10 if the last digit is 0. |
| 11 | No (remainder 2) | 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 640 a prime number?
No. 640 = × 5, so it is divisible by 2.
What are the factors of 640?
1, 2, 4, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 320, 640.
What is 640 in binary?
1010000000 (hexadecimal 280, octal 1200).
What is the square root of 640?
About 25.2982, exactly 8√10.
What is 640 in Roman numerals?
DCXL.
Is 640 divisible by 3?
No. Its digits add up to 10, which is not a multiple of 3.
What is special about the number 640?
640 is a Harshad number: divisible by its digit sum 10.