Is 90 a prime number?
90 is not a prime number. Here is the quickest way to see it.
The answer
No. 90 is not a prime number: 2 × 45 = 90.
How to see that 90 is not prime
90 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 90.
90 has 12 factors (1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45 and 90), which make 6 factor pairs.
Split 90 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 90 = 2 × 45
- 90 = 2 × 3 × 15
- 90 = 2 × 3 × 3 × 5
With the repeated primes written as powers, that is 2 × 3² × 5.
90 in the times tables
90 turns up in 2 different times tables, the 9 and 10: 9 × 10 and 10 × 9.
Quick checks that catch 90
- The last digit, 0, is even, so 2 goes in.
- The digits add up to 9, which is in the 3 times table, so 3 goes in.
- It ends in 0, so 5 goes in.
- It is even and passes the check for 3 as well, so 6 goes in.
- The digit total, 9, is a multiple of 9, so 9 goes in.
- It ends in 0, so 10 goes in.
90 at a glance
- In words
- ninety
- In Roman numerals
- XC
- Odd or even
- Even: it ends in 0
- Multiple of 10
- 9 tens
- Its factors add up to
- 234, or 144 without 90 itself, which makes 90 an abundant number
The numbers either side of 90
- 89 is prime: 2, 3, 5 and 7 do not go into it, and 11 × 11 is already past 89. Check 89 in full
- 91 is not prime. 91 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 × 13 = 91. Check 91 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 90?
89 below it and 97 above it, a gap of 8.
What is 90 squared?
90 × 90 = 8100.
What is the smallest number with as many factors as 90?
60. It has 12 factors, the same as 90.
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.