Mathematics · Greatest common factor
GCF of 15 and 20
The greatest common factor of 15 and 20 is 5. It is the largest whole number that divides both 15 and 20 with no remainder.
You will also see it written as the highest common factor (HCF) or the greatest common divisor (GCD) of 15 and 20. All three names mean the same thing.
Greatest common factor
5
GCF, GCD or HCF of 15 and 20
Least common multiple
60
LCM of 15 and 20
15/20 simplified
3/4
both divided by 5
Below is the same answer reached three different ways, because different classes teach different methods. Every line is worked out for these two numbers, so you can copy the one your teacher asks for.
Listing the factors of 15 and 20
Factors of 15: 1, 3, 5, 15
Factors of 20: 1, 2, 4, 5, 10, 20
Common factors: 1, 5
Greatest of those = 5
The common factors of 15 and 20 are 1 and 5, and that is the complete list. The largest one, 5, is the GCF.
Prime factorization method
15 = 3 × 5 (3 × 5)
20 = 22 × 5 (2 × 2 × 5)
Shared primes at their lowest power: 5
GCF = 5
Only 5 appears in both lists, and only once in each, so nothing else carries over and the GCF is 5.
Euclid's algorithm, step by step
20 ÷ 15 = 1 remainder 5
15 ÷ 5 = 3 remainder 0
Last non-zero remainder = 5, so the GCF is 5
Divide the larger number by the smaller, then keep dividing the previous divisor by the remainder. When the remainder reaches zero, the divisor you used last is the GCF. It took 2 steps for 15 and 20.
LCM of 15 and 20, and the fraction 15/20
For any two numbers, the GCF multiplied by the LCM equals the two numbers multiplied together. That gives a quick way to get one from the other, and a way to check both.
GCF × LCM = 5 × 60 = 300
15 × 20 = 300
They match, so both answers are right
Dividing the top and bottom of 15/20 by 5 reduces it to 3/4, which is as far as it goes. Anything smaller than the GCF would leave the fraction reducible.
GCF of 15 and 20: FAQs
What is the GCF of 15 and 20?
The GCF of 15 and 20 is 5. It is the largest whole number that divides both of them exactly.
What are the common factors of 15 and 20?
The common factors of 15 and 20 are 1 and 5. The greatest of them is 5.
Is the GCF of 20 and 15 the same as the GCF of 15 and 20?
Yes. The order makes no difference. The greatest common factor of 20 and 15 is 5, exactly the same as the GCF of 15 and 20, because you are looking at the same two numbers either way. The same is true if you see it written as the highest common factor of 20 and 15.
What is the LCM of 15 and 20?
The LCM of 15 and 20 is 60. You can check it with GCF times LCM: 5 times 60 equals 300, which is the same as 15 times 20.
Are 15 and 20 coprime?
No. 15 and 20 share 5 as a common factor, so they are not coprime. Coprime numbers have a GCF of exactly 1.
How do you simplify 15/20?
Divide the top and the bottom by their GCF, which is 5. That gives 15 divided by 5 equals 3, and 20 divided by 5 equals 4, so 15/20 simplifies to 3/4.
Which method is quickest for 15 and 20?
Listing factors, because both numbers are small enough to write the factors out by eye.
Nearby pairs
Any other numbers? The GCF Calculator takes two to ten values and shows the same three methods, and the math calculators page has the rest of the set.
Open the GCF Calculator →
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