The first, second, and third terms of a proportion are 2x, 3x, and 12, respectively. The fourth proportional is y.

Use the property that the product of extremes equals the product of means: $(\text{First} \times \text{Fourth}) = (\text{Second} \times \text{Third})$. Here, $(2x \times y) = (3x \times 12)$.

Students often fail to cancel out the variable $x$ on both sides, leading to confusion or incorrect expressions involving $x$ in the final answer.

This concept of fourth proportionals is foundational for understanding 'Variation' (Direct/Inverse), which is frequently tested in speed-time-distance problems.