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What is the greatest common factor of 42, 63, and 105?
is ##21##
Factors of ##42## are ##{1,2,3,6,7,14,21,42}##
Factors of ##63## are ##{1,3,7,9,21,63}##
Factors of ##105## are ##{1,3,5,7,15,21,35,105}##
Common factors are just ##{1,3,7,21}## and
Greatest Common Factor is ##21##.