20% of A is equal to 30% of B.

To find the ratio A:B when $x\% of A = y\% of B$, simply invert the numbers: $A:B = y:x$. Here, $20:30$ becomes $30:20$, which simplifies to $3:2$.

Many students mistake the ratio to be $2:3$ by directly reading the numbers in the question, forgetting that for $A=B$ to hold, $A$ must be larger than $B$ because its coefficient is smaller.

This concept of inverse proportionality ($A \cdot x = B \cdot y \implies A:B = y:x$) is fundamental in Time and Work problems where efficiency and time are inversely related.