Time taken by A and B together (A+B) = 12 days. Time taken by A alone = 20 days.

Use the formula $\frac{x \times y}{y - x}$, where $x$ is the time taken by A alone and $y$ is the time taken by both together. Here, $x=20$ and $y=12$.

Students often add the days instead of finding the difference, incorrectly calculating $20 + 12 = 32$ or getting confused with the reciprocal rule $\frac{1}{B} = \frac{1}{12} - \frac{1}{20}$ and failing to invert the result at the final step.

This inverse relationship between work and time is identical to the concept of Pipe and Cistern problems, where the inflow and outflow rates determine the net filling time.