Prime and composite numbers · Grade 5
In Prime and composite numbers (Grade 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.
Not on the site yet
Parts of this topic that are not built yet: Writing a number as a product of primes.
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 new Israeli elementary programme (2023).