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           Hi Fadwa. 
          You can always find the GCF if you break the two numbers down to their prime factors. A fundamental theorem holds that each number has only one prime factorization. 
          For example, the prime factorization of 1680 is 24  3 5 7 and the prime factorization of 6048 is 25  33  7 . 
          The GCF of a pair of numbers is always the product of the prime factors they have in common. 
          Hope this helps, 
            Stephen La Rocque. 
              
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