Given proportion: 8 : 4 : : 3.2 : x

In a proportion $a: b:: c: d$, the product of extremes equals the product of means ($a \times x = b \times c$). Also, notice that $b$ is half of $a$ ($4 = 8 / 2$), so $x$ must be half of $3.2$, which is $1.6$.

Multiplying the wrong terms or dividing by $3.2$ instead of $8$. Always equate (Product of Extremes) = (Product of Means).

Proportion concepts are fundamental in solving unitary method problems, mixtures, and scale factor problems in quantitative aptitude.