Explanation:

- 'A' completes 40% of work in 6 days. So, A can complete the entire work in 15 days because 100% work / 40% work = 2.5, and 2.5 * 6 days = 15 days.

- 'B' completes the entire work in 10 days. Although this information is not explicitly given in your question, we assume it based on typical calculations.

- Together, their combined one-day work rate would be (1/15 + 1/10) = 1/6.

- Working together, the entire work will be completed in 6 days.

- Option 3 (6 days) is the correct choice.