Direct variation is y = kx. If x doubles, y doubles. Find k from one pair, then use it on the next. Ratios as a set are in fractions and ratios.
One pair makes k
y = 12 when x = 3 gives k = 4, so y = 4x. Inverse variation is the product staying constant, in inverse variation.
The short list is in ACT Math formulas.
The bottom line
y = kx. Same ratio, not the same product.
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