Is 20 a prime number?
20 is not a prime number. Here is the quickest way to see it.
The answer
No. 20 is not a prime number: 2 × 10 = 20.
How to see that 20 is not prime
20 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 20.
20 has 6 factors (1, 2, 4, 5, 10 and 20), which make 3 factor pairs.
Split 20 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 20 = 2 × 10
- 20 = 2 × 2 × 5
With the repeated primes written as powers, that is 2² × 5.
20 in the times tables
20 turns up in 4 different times tables, the 2, 4, 5 and 10: 2 × 10, 4 × 5, 5 × 4 and 10 × 2.
Quick checks that catch 20
- The last digit, 0, is even, so 2 goes in.
- Half of it is 10, which is still even, so 4 goes in.
- It ends in 0, so 5 goes in.
- It ends in 0, so 10 goes in.
20 at a glance
- In words
- twenty
- In Roman numerals
- XX
- Odd or even
- Even: it ends in 0
- Multiple of 10
- 2 tens
- Its factors add up to
- 42, or 22 without 20 itself, which makes 20 an abundant number
The numbers either side of 20
- 19 is prime: 2 and 3 do not go into it, and 5 × 5 is already past 19. Check 19 in full
- 21 is not prime. Its digits add up to 2 + 1 = 3, which is a multiple of 3. That means 21 is a multiple of 3 too, so 3 is a factor. Check 21 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 20?
19 below it and 23 above it, a gap of 4.
What is 20 squared?
20 × 20 = 400.
What is the smallest number with as many factors as 20?
12. It has 6 factors, the same as 20.
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.