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Common factors are just the factors that two or more numbers have in common.  (These are also called "common divisors.")

Let's just do one:

Find the common factors of

24 and 36

1 )List the factors of each:
24 :  1 , 2 , 3 , 4 , 6 , 8 , 12 , 24
36 :  1 , 2 , 3 , 4 , 6 , 9 , 12 , 18 , 36

 

2 )Circle the factors that are in both lists:
24 :  ( 1 ) , ( 2 ) , ( 3 ) , ( 4 ) , ( 6 ) , 8 , ( 12 ) , 24
36 :  ( 1 ) , ( 2 ) ,  ( 3 ) , ( 4 ) , ( 6 ) , 9 , ( 12 ) , 18 , 36

 

The common factors of 24and 36are:

1 , 2 , 3 , 4 , 6 , 12

 

Here's another one:

Find the common factors of

18 and 25
 

18 :  ( 1 ) , 2 , 3 , 6 , 9 , 18
25 :  ( 1 ) , 5 , 25

 

The common factor of 18and 25is:

1