The number 78
78 is a triangular number: 1 + 2 + … + 12 = 78. Its prime factorisation is 2 × 3 × 13. Divisors (8): 1, 2, 3, 6, 13, 26, 39, 78.
Pick any whole number from 1 to 100,000.
How to work it out
- 1
What is special about 78: a triangular number: 1 + 2 + … + 12 = 78.
- 2
78 is composite. Its prime factorisation is
- 3
The divisors add up to 168; without 78 itself that is 90, so 78 is abundant (90 is more than 78). Euler's totient φ(78) = 24 numbers up to 78 share no factor with it.
- 4
The digits of 78 add up to 15, and repeating that until one digit is left gives the digital root 6. The Collatz sequence (halve if even, else 3n+1) takes 35 steps to reach 1.
LaTeX source of the formulas on this page
\[ 78 = 2 \times 3 \times 13 \]
78 at a glance
| Property | Value |
|---|---|
| Type | composite |
| Number of divisors | 8 |
| Sum of divisors | 168 |
| Euler totient φ(n) | 24 |
| Square | 6084 |
| Cube | 474552 |
| Square root | 8.831761 |
| Digit sum / digital root | 15 / 6 |
| Collatz steps to 1 | 35 |
| Scientific notation | 7.8 × |
78 in other forms
| Form | Value |
|---|---|
| In words | seventy-eight |
| Roman numerals | LXXVIII |
| Binary (base 2) | 1001110 |
| Octal (base 8) | 116 |
| Hexadecimal (base 16) | 4E |
| Base 3 | 2220 |
| As time | 78 seconds = 1 min 18 s |
Is 78 divisible by 2 to 12?
| Divisor | Answer | Rule |
|---|---|---|
| 2 | Yes (78 ÷ 2 = 39) | A number is divisible by 2 if the last digit is even (0, 2, 4, 6, 8). |
| 3 | Yes (78 ÷ 3 = 26) | A number is divisible by 3 if the digits add up to a multiple of 3. |
| 4 | No (remainder 2) | 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 | Yes (78 ÷ 6 = 13) | A number is divisible by 6 if it is divisible by both 2 and 3. |
| 7 | No (remainder 1) | 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 6) | A number is divisible by 8 if the last three digits make a multiple of 8. |
| 9 | No (remainder 6) | 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 1) | A number is divisible by 11 if the alternating sum of the digits is a multiple of 11. |
| 12 | No (remainder 6) | A number is divisible by 12 if it is divisible by both 3 and 4. |
Questions
Is 78 a prime number?
No. 78 = 2 × 3 × 13, so it is divisible by 2.
What are the factors of 78?
1, 2, 3, 6, 13, 26, 39, 78.
What is 78 in binary?
1001110 (hexadecimal 4E, octal 116).
What is the square root of 78?
About 8.8318.
What is 78 in Roman numerals?
LXXVIII.
Is 78 divisible by 3?
Yes. Its digits add up to 15, a multiple of 3.
What is special about the number 78?
78 is a triangular number: 1 + 2 + … + 12 = 78.