GCF for 7, 15 and 21

"Greatest Common Factor of 3 Numbers" Calculator
Greatest Common Factor of
,
and

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

Answer

GCF of 7, 15 and 21 is
1

(One)

1 is the largest positive integer that divides 7, 15 and 21 without a remainder — every other common divisor of the three divides it too.

Finding the GCF of 7, 15 and 21 by Prime Factorization

Split all three numbers into prime factors, then keep every prime that appears in all three of them, each taken as many times as it appears in the poorest factorization. Multiplying what is left gives the greatest common factor:

All Prime Factors of 7: 7 (exponent form: 7)

All Prime Factors of 15: 3 × 5 (exponent form: 3 × 5)

All Prime Factors of 21: 3 × 7 (exponent form: 3 × 7)

Prime factor71521Lowest power
3—33—
5—5——
77—7—

Not one prime factor is shared by all three numbers, which makes 7, 15 and 21 relatively prime as a set. The only positive integer dividing all of them is 1.

Checking the GCF of 7, 15 and 21 Two Numbers at a Time

The greatest common factor is associative, so three numbers can be handled as two pairs — and this is what makes the Euclidean algorithm usable here: run it on the first pair, then run it again on that result and the third number.

GCF(a, b, c) = GCF(GCF(a, b), c)

GCF(7, 15) = 1

GCF(1, 21) = 1

GCF(7, 15, 21) = 1

The answer checks out by division — all three quotients come out whole:

7 ÷ 1 = 7

15 ÷ 1 = 15

21 ÷ 1 = 21

7, 15 and 21 have no common factor left — that is what makes 1 the greatest one.

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See Also

About "Greatest Common Factor of 3 Numbers" Calculator

This calculator finds the Greatest Common Factor (GCF) of three numbers at once, without doing it in two passes. For example, it can help you find out what's the Greatest Common Factor (GCF) of 7, 15 and 21? (The answer is: 1). Enter the first number (e.g. '7'), the second (e.g. '15') and the third (e.g. '21'), then hit the 'Calculate' button.
The Greatest Common Factor of three numbers — also called the greatest common divisor (GCD) or highest common factor (HCF) — is the largest positive integer that divides all three of them with zero remainder
"Greatest Common Factor of 3 Numbers" Calculator
Greatest Common Factor of
,
and

GCF Table

Number 1Number 2Number 3GCF
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, and it is the check this page prints. 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.

See Also