GCF for 15 and 21
What is the Greatest Common Divisor of 15 and 21?
Answer
(Three)
Finding GCF for 15 and 21 using all factors (divisors) listing
The first method to find GCF for numbers 15 and 21 is to list all factors for both numbers and pick the highest common one:
All factors of 15: 1, 3, 5, 15
All factors of 21: 1, 3, 7, 21
So the Greatest Common Factor for 15 and 21 is 3
Finding GCF for 15 and 21 by Prime Factorization
The second method to find GCF for numbers 15 and 21 is to list all Prime Factors for both numbers and multiply the common ones:
All Prime Factors of 15: 3, 5
All Prime Factors of 21: 3, 7
As we can see there is only one Prime Factor common to both numbers. It is 3
So 3 is the Greatest Common Factor of 15 and 21
Related Calculations
See Also
- Least Common Multiple - Find the Least Common Multiple (LCM) of two numbers
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GCF Table
About "Greatest Common Factor" Calculator
Greatest Common Factor (GCF) also known as the Greatest Common Divisor (GCD) or Highest Common Factor (HCF) - it is the largest positive integer that divides each of the integers with zero remainder.