Prime Number Sieve
Class 6 Maths: playing with numbers (primes and the Sieve of Eratosthenes)
Cross out the multiples of 2, 3, 5 and 7 on a 1 to 50 grid. The numbers that survive are the prime numbers.
Words to know
- Factor
- A number that divides another number exactly. 3 is a factor of 12.
- Multiple
- A number you get by multiplying a number by 1, 2, 3 and so on. 12 is a multiple of 3.
- Prime number
- A number with exactly two factors: 1 and itself, like 2, 3, 5, 7.
- Composite number
- A number with more than two factors, like 4, 6, 9.
- Sieve of Eratosthenes
- A way to find primes by crossing out the multiples of each prime in turn.
Quick check
Answer 3 questions to check what you learned.
Try at home
Write 1 to 100 on squared paper and use the sieve. How many primes are there up to 100? (Answer: 25.)