Explanation:

Let's evaluate the information from the statements:

- Statement I: "10 men and 8 women are working on a job. How long will they take to finish the work?"

- This provides information about the workforce but not the time required for any specific work.

- Statement II: "20 men and 4 women would have taken 25 days."

- This provides a specific time and workforce combination, which helps find how many work units are done per day.

To find how long 20 men and 8 women will take:

- Alone, both statements are insufficient; they don't directly give the number of days needed for 20 men and 8 women.

- Together, you can use the work rate from Statement II to assist in evaluating with the workforce in Statement I, leading to a solution.

Hence, the correct option is: Option 3: If questions can be answered with the help of both statements together, but not with the help of either statement alone.