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To get the Greates Common Factor (GCF) of 1638 and 819 we need to factor each value first and then we choose all the copies of factors and multiply them:
|819: || ||3||3||7||13|
|GCF: || ||3||3||7||13|
The Greates Common Factor (GCF) is: 3 x 3 x 7 x 13 = 819