Explanation: Factors of 20 are 1, 2, 4, 5, 10 and 20. Factors of 35 are 1, 5, 7 and 35. You may notice that two factors of 20 are also factors of 35, they are 1 and 5. These 2 numbers are called 'common factors' of 20 and 35. The largest of the two numbers is 5, it is therefore the 'greatest common factor'. It is known as the GCF. Identifying the GCF helps when you are working with fractions.

Examples:

Find the GCF of 8 and 9. Factors of 8 are 1, 2, 4 and 8. Factors of 9 are 1, 3 and 9. The GCF = 1.

Find the GCF of 5 and 20. Factors of 5 are 1 & 5. Factors of 20 are 1, 2, 4, 5, 10 & 20 The GCF = 5

Directions: Analyze each of the two given numbers and identify the factors of each number. Then identify the GCF for the two given numbers. Strive for accuracy and then strive to increase your speed. Good luck and enjoy the challenge!

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