Prime and Composite Numbers (Grade 4)
A prime number has exactly two factors: 1 and itself (like 7 — only 1 and 7 divide it). A composite number has more than two factors (like 12). The number 1 is neither prime nor composite (it has only one factor). To test a number, check whether any number from 2 up to its square root divides it evenly — if one does, it's composite; if none do, it's prime.
Understanding prime and composite numbers
A prime number has exactly two factors: 1 and itself (like 7 — only 1 and 7 divide it). A composite number has more than two factors (like 12). The number 1 is neither prime nor composite (it has only one factor). To test a number, check whether any number from 2 up to its square root divides it evenly — if one does, it's composite; if none do, it's prime.
Key Idea
A prime number has exactly two factors: 1 and itself (like 7 — only 1 and 7 divide it). A composite number has more than two factors (like 12). The number 1 is neither prime nor composite (it has only one factor). To test a number, check whether any number from 2 up to its square root divides it evenly — if one does, it's composite; if none do, it's prime.
Seeing it in action
Worked example
Is 7 prime or composite?
Check divisors up to √7 (about 2.6): does 2 divide 7? No. → no factor besides 1 and 7. → Prime.
21 has an inside factor pair: 3 × 7.
Worked example 2
Is 21 prime or composite?
3 divides 21 (3×7 = 21). → it has a factor besides 1 and itself. → Composite.
Try a few
Is 13 prime?
Is 15 prime?
3 × 5.
Is 9 prime?
3 × 3.
Factor Fields
A calm number-theory game for factors, multiples, primes, GCF, and LCM.
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