Explanation:

- A completes the entire work in 30 days.

- A's 10-day work equals $\frac{10}{30}=\frac{1}{3}$ of the job.

- So, $\frac{2}{3}$ of the work is left, finished by B in 20 days.

- B completes $\frac{2}{3}$ of work in 20 days, meaning B takes 30 days for full work equivalently.

- B's one day work’s efficiency = $\frac{1}{30}$.

- A's one day work’s efficiency = $\frac{1}{30}$.

- Together, A and B's efficiency = $\frac{1}{30}+\frac{1}{30}=\frac{1}{15}$.

- They finish work in $\frac{1}{\frac{1}{15}}$, leading to 15 days.

- So, Option: 2 - 15 days is the correct answer.

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