Prime and composite numbers · Year 5
In Prime and composite numbers (Year 5) you practise 2 skills: Prime or composite · Factors and multiples. A prime number has exactly two factors: 1 and itself. A number with more than two factors is composite. The number 1 is neither prime nor composite, because it has only one factor.
To prove that a number is composite, it is enough to find one factor other than 1 and the number itself. For example, 207 = 23 × 9, so 207 is composite. 2 is the only even prime: every other even number also divides by 2. On the 1 to 100 board you can find the primes: colour the multiples of 2, 3, 5 and 7, and the numbers left uncoloured are the primes. There is no greatest prime: the primes never run out.
Tip: Before you decide that a number is prime, check whether it divides by 2, 3, 5 or 7.
What you learn
Prime or composite
For example: Is 9 a prime number?
- No
- Yes
Answer and solution
The answer: No
9 divides by 3: 9 = 3 × 3 It has more than two factors, so it is composite, not prime. The answer: no.
Factors and multiples
For example: How many factors does 58 have?
Answer and solution
The answer: 4
The factors of 58: 1, 2, 29, 58 That is 4 factors.
The exercises are made by rules and the answers are worked out in code, so there are new exercises every time. The topic follows the national curriculum in England: mathematics (key stages 3 and 4).