Is 77 a prime number?
77 is not a prime number. Here is the quickest way to see it.
The answer
No. 77 is not a prime number: 7 × 11 = 77.
How to see that 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.
Up to 100, the only composite numbers that pass the checks for 2, 3 and 5 are 49, 77 and 91, and 7 goes into all three.
77 has four factors: 1 and 77, plus one pair in between, 7 and 11.
Why 77 looks prime
77 has two matching digits, and every two-digit number with matching digits is in the 11 times table: 77 = 11 × 7. So 22, 33, 44, 55, 66, 77, 88 and 99 are none of them prime.
Compare 73, the prime just below it. Try the same checks: 73 ÷ 2 = 36 remainder 1, 73 ÷ 3 = 24 remainder 1, 73 ÷ 5 = 14 remainder 3 and 73 ÷ 7 = 10 remainder 3. None goes in exactly, and 11 × 11 = 121 is more than 73, so 73 really is prime.
Split 77 into prime factors
77 splits straight into two primes, 7 × 11, so its factor tree has only one branch.
77 in the times tables
77 turns up in 2 different times tables, the 7 and 11: 7 × 11 and 11 × 7.
Quick checks that catch 77
- Its two digits match, and every two-digit number like that is in the 11 times table.
77 at a glance
- In words
- seventy-seven
- In Roman numerals
- LXXVII
- Odd or even
- Odd: it ends in 7
- Its factors add up to
- 96, or 19 without 77 itself, which makes 77 a deficient number
The numbers either side of 77
- 76 is not prime. 76 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 76. Check 76 in full
- 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. Check 78 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 77?
73 below it and 79 above it, a gap of 6.
What is 77 squared?
77 × 77 = 5929.
What is the smallest number with as many factors as 77?
6. It has 4 factors, the same as 77.
Number facts are the base of 11+ maths
Factors, multiples, primes and square numbers come up all over 11+ maths, from simplifying fractions to ratio. The free diagnostic checks maths, English and both kinds of reasoning, and shows what to practise next. No card is required.