Primes: exactly two factors
Count the factors. 7 has two: 1 and 7. 8 has four.
Step 1 of 4
- Numbers 2 to 21 in a grid.
- Cross out multiples of 2 (keep 2).
- Cross out multiples of 3 (keep 3).
- What is left: the primes.
The rule
A prime has exactly two factors: 1 and itself. 7 is prime — only 1 and 7 divide it. 6 is not — it has four factors.
Watch out!
1 is not prime. For 1, "itself" and "1" are the same number. It has only one factor.
2 is prime too — its only factors are 1 and 2. It is the only even prime. Every other even number has the factors 1, 2 and itself. That is more than two factors.
Cross out the multiples of 2, 3 and 5 in the lab.
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