6/12 and 4/8 are equal fractions, as, when simplified, they share a simplified fraction.
Note that what you do to the denominator, you do to the numerator. Find common denominators for both fractions:
(4/8)/(2/2) = 2/4
(6/12)/(3/3) = 2/4
As you can tell, when they share a common denominator (4), the numerators are the same as well (3).
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