GCF for 7, 15 and 21

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Greatest Common Factor of
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What's the Greatest Common Factor (GCF) of 7, 15 and 21?

Answer

GCF of 7, 15 and 21 is
1

Common factors: 1

Only 1 divides all three: no prime is a factor of each of 7, 15 and 21.

Pairs do share factors — GCF(7, 21) = 7, GCF(15, 21) = 3 — just none greater than 1 common to all three.

7 — prime

15 = 3 × 5

21 = 3 × 7

Prime71521Lowest power
3—33—
5—5——
77—7—

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

GCF = 1

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GCF Table

1st2nd3rdGCF
67151
77151
78151
79151
710151
711151
712151
713151
714151
715151
715161
715171
715181
715191
715201
715211
715221
715231
715241
715251
715261
715271
715281
715291
715301
715311
715321
715331
715341
715351

FAQ

What's the Greatest Common Factor (GCF) of 7, 15 and 21?

GCF of 7, 15 and 21 is 1

What prime factors do 7, 15 and 21 share?

None at all: 7, 15 and 21 have no prime factor in common, so the only positive integer dividing all three is 1 and their GCF is 1.

Can you get the GCF of 7, 15 and 21 two numbers at a time?

Yes. The GCF of 7 and 15 is 1, and the GCF of 1 and 21 is 1 — the same answer, because the greatest common factor is associative and does not depend on the order of the three numbers.

How does the GCF simplify the ratio 7 : 15 : 21?

Divide every term by the GCF 1: 7 ÷ 1 = 7, 15 ÷ 1 = 15 and 21 ÷ 1 = 21. So 7 : 15 : 21 in lowest terms is 7 : 15 : 21, and no whole number other than 1 divides all three of those.

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