ACT Math · Ratios, rates, and percents

The variable y varies inversely with x. If y = 8 when x = 3, what is y when x = 6?

The variable yy varies inversely with xx. If y=8y = 8 when x=3x = 3, what is yy when x=6x = 6?

Answer and explanation

Answer: G: 4

Inverse variation means y=kxy=\frac{k}{x}, so the product xyxy stays constant. From the given values, k=8⋅3=24k=8\cdot3=24. When x=6x=6, y=246=4y=\frac{24}{6}=4, choice G. Doubling xx from 33 to 66 cuts yy in half, from 88 to 44. Choice F (1616) treats the relationship as direct variation, doubling yy when xx doubles. Choice H (22) divides yy by 44, dividing twice instead of once. Choice J (2424) reports the constant kk itself instead of the value of yy. A quick check: 6⋅4=24=3⋅86\cdot4=24=3\cdot8, so the product is unchanged.

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