Direct variation is y = kx. Inverse variation is y = k/x. Find k from one pair, then use it on the next pair. This is algebra, not a Science graph. The graph version is in direct versus inverse on Science graphs.
Solve for k first
If y varies directly with x, and y = 10 when x = 2, then k = 5, so y = 5x. If y varies inversely with x, the same pair gives k = 20, so y = 20/x. Proportions that are direct variation in disguise are in proportions.
A scatterplot can look direct or inverse without being a clean y = kx, in scatterplots and correlation. Functions more generally are in ACT Math functions.
The bottom line
Direct: multiply by k. Inverse: divide k by x. Get k from the given pair before you plug in the new x.
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