GCF (HCF) of 15 and 21

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Greatest Common Factor of
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What is the Greatest Common Divisor of 15 and 21?

Answer

GCF (HCF) of 15 and 21 is
3

Common factors: 1, 3

3 divides both: 15 ÷ 3 = 5, 21 ÷ 3 = 7 — and 5 and 7 share no factor but 1, so no larger number divides both.

15 = 3 × 5

21 = 3 × 7

Prime1521Lowest power
3333
55——
7—7—

— = power 0: the prime is not a factor of this number, so it cannot be in the GCF.

GCF = 3

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GCF Table

Number 1Number 2GCF
1555
1563
1593
15105
15123
15161
15171
15183
15205
15213
15221
15231
15243
15255
15261
15273
15281
15291
153015
15321
15333
15355
15363
15381
15405
15423
15441
154515
15483
15491

FAQ

What is the Greatest Common Divisor of 15 and 21?

GCF (HCF) of 15 and 21 is 3

How to find the GCF of 15 and 21?

You can find the GCF of 15 and 21 by listing all factors of both numbers and picking the greatest common one, or by prime factorization. The GCF of 15 and 21 is 3.

What prime factors do 15 and 21 share?

The prime factorizations of 15 and 21 have 3 in common, and multiplying those shared primes gives 3 — exactly the greatest common factor.

How does the GCF reduce the fraction 15/21?

Divide the numerator and the denominator by the GCF 3: 15 ÷ 3 = 5 and 21 ÷ 3 = 7. So 15/21 in lowest terms is 5/7.

See Also