Explanation:

- A can do the entire work in 20 days. So, A completes 1/20 of the work in a day.

- B can finish the same work in 15 days. So, B completes 1/15 of the work in a day.

- B worked for 9 days, completing 9 * (1/15) = 9/15 = 3/5 of the work.

- This leaves 1 - 3/5 = 2/5 of the work remaining for A to finish.

- A can finish 1/20 of the work in one day, so to finish 2/5 of the work, A will need:

$$ \text{Days} = (2/5) \div (1/20) = 8 \text{ days} $$

- Option:1, 10 days – This is more than required.

- Option:2, 8 days – This is the correct calculation.

- Option:3, 9 days – This is more than needed.

- Option:4, 7 days – This is less than necessary.

- Option 2, 8 days is the correct answer.