Download Lesson 2 Reteach Greatest Common Factor

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Date
Lesson 2
Reteach
Greatest Common Factor
The greatest common factor (GCF) of two or more numbers is the
greatest of the common factors of the numbers. To find the GCF,
you can make a list or use prime factorization.
Find the GCF of 12 and 30.
Make an organized list of the factors for each number.
12: 1, 2, 3, 4, 6, 12
30: 1, 2, 3, 5, 6, 10, 15, 30
The common factors are 1, 2, 3, and 6. The greatest is 6.
So, the GCF of 12 and 30 is 6.
Find the GCF of 18 and 27 by using prime factorization.
Write the prime factorizations of 18 and 27.
18
2
2
×
×
3
27
9
×
3
3
3
×
×
3
9
×
3
The common prime factors are 3 and 3.
So, the GCF of 18 and 27 is 3 × 3 or 9.
Identify the common factors of each set of numbers.
1. 8, 12
2. 10, 15
3. 81, 27
4. 15, 20
Find the GCF of each set of numbers.
5. 6, 12
7.
62
44, 55
6. 28, 42
8. 35, 20, 15
Grade 5 • Chapter 8 Fractions and Decimals