Is 78 a prime number?
78 is not a prime number. Here is a way to check it.
The answer
No. 78 is not a prime number: 2 × 39 = 78.
How to see that 78 is not prime
78 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 78.
78 has 8 factors (1, 2, 3, 6, 13, 26, 39 and 78), which make 4 factor pairs.
Split 78 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 78 = 2 × 39
- 78 = 2 × 3 × 13
78 in the times tables
No fact from the 2 to 12 times tables, up to × 12, makes 78. Its smallest pair, 2 × 39, needs the 39 times table, which goes past 12.
Quick checks that catch 78
- The last digit, 8, is even, so 2 goes in.
- The digits add up to 15, which is in the 3 times table, so 3 goes in.
- It is even and passes the check for 3 as well, so 6 goes in.
78 at a glance
Optional extra number facts:
- In words
- seventy-eight
- In Roman numerals
- LXXVIII
- Odd or even
- Even: it ends in 8
- Triangular number
- 1 + 2 + … + 12 = 78, a triangle of counters with 12 on the bottom row
- Its factors add up to
- 168, or 90 without 78 itself. 78 is abundant: Abundant means this sum is greater than the number.
The numbers either side of 78
- 77 is not prime. 77 is odd, its digits do not add up to a multiple of 3, and it does not end in 5, so the quick checks for 2, 3 and 5 all fail. Try 7 next: 7 × 11 = 77. Check 77 in full
- 79 is prime: 2, 3, 5 and 7 do not go into it, and 11 × 11 is already past 79. Check 79 in full
Turn the digits round
78 backwards is 87. 87 is not prime. Its digits add up to 8 + 7 = 15, which is a multiple of 3. That means 87 is a multiple of 3 too, so 3 is a factor. Check 87 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 78?
73 below it and 79 above it, a gap of 6.
What is 78 squared?
78 × 78 = 6084.
What is the smallest number with as many factors as 78?
24. It has 8 factors, the same as 78.
Continue your number practice
Explore the diagnostic for another set of practice questions.