\[20,30,100\]
Method 1: By Listing Factors
1
List the factors of each number.
Factors of 20
: 1, 2, 4, 5, 10, 20
Factors of 30
: 1, 2, 3, 5, 6, 10, 15, 30
Factors of 100
: 1, 2, 4, 5, 10, 20, 25, 50, 100
2
Find the largest number that is shared by all rows above. This is the GCF.
GCF = 1010

Method 2: By Prime Factors
1
List the prime factors of each number.
Prime Factors of 20
: 2, 2, 5
Prime Factors of 30
: 2, 3, 5
Prime Factors of 100
: 2, 2, 5, 5
2
Find the intersection of these primes.
2,52,5

3
Multiply these numbers: 2×5=102\times 5=10. This is the GCF.
GCF = 1010

Done
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