Is 84 a prime number?
84 is not a prime number. Here is the quickest way to see it.
The answer
No. 84 is not a prime number: 2 × 42 = 84.
How to see that 84 is not prime
84 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 84.
84 has 12 factors (1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42 and 84), which make 6 factor pairs.
Split 84 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 84 = 2 × 42
- 84 = 2 × 2 × 21
- 84 = 2 × 2 × 3 × 7
With the repeated primes written as powers, that is 2² × 3 × 7.
84 in the times tables
84 turns up in 2 different times tables, the 7 and 12: 7 × 12 and 12 × 7.
Quick checks that catch 84
- The last digit, 4, is even, so 2 goes in.
- The digits add up to 12, which is in the 3 times table, so 3 goes in.
- Half of it is 42, which is still even, so 4 goes in.
- It is even and passes the check for 3 as well, so 6 goes in.
84 at a glance
- In words
- eighty-four
- In Roman numerals
- LXXXIV
- Odd or even
- Even: it ends in 4
- Its factors add up to
- 224, or 140 without 84 itself, which makes 84 an abundant number
The numbers either side of 84
- 83 is prime: 2, 3, 5 and 7 do not go into it, and 11 × 11 is already past 83. Check 83 in full
- 85 is not prime. 85 ends in 5, and every number ending in 5 or 0 is in the 5 times table, so 5 is a factor. Check 85 in full
Turn the digits round
84 backwards is 48. 48 is not prime. 48 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 48. Check 48 in full
Related pages
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Questions parents ask
Which prime numbers are either side of 84?
83 below it and 89 above it, a gap of 6.
What is 84 squared?
84 × 84 = 7056.
What is the smallest number with as many factors as 84?
60. It has 12 factors, the same as 84.
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.