Explanation:

- A completes the entire work in 15 days. Therefore, A does $\frac{1}{15}$ of the work in one day.

- B completes the work in 20 days. Therefore, B does $\frac{1}{20}$ of the work in one day.

- Working together, A and B complete $\frac{1}{15}+\frac{1}{20}=\frac{4}{60}+\frac{3}{60}=\frac{7}{60}$ of the work in one day.

- To find out how long they take to complete 35% (or $\frac{35}{100}$) of the work: $\frac{35}{100}=\frac{7}{20}$.

- Let $x$ be the days required:

$$

x \times \left(\frac{7}{60}\right) = \frac{7}{20}

$$

- Solving for $x$:

$$

x = \frac{7}{20} \times \frac{60}{7} = 3

$$

- Therefore, they complete 35% of the work in 3 days.

- Option 2 is correct.