If two fractions are proportional, what you do to the denominator, you must do also to the numerator.
In this case, 3/7 and 12/21 is non-proportional, because the numerator was multiplied by 4 (3 x 4 = 12) , while the denominator was multiplied by 3 (7 x 3 = 21). If it was proportional, both the numerator and denominator must be multiplied by either 3 or 4.
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