Explanation:

- 'A' can complete the work in 15 days, so 'A' does 1/15 of the work in a day.

- 'B' can complete the work in 20 days, so 'B' does 1/20 of the work in a day.

- Together, 'A' and 'B' can do $\frac{1}{15}+\frac{1}{20}=\frac{4+3}{60}=\frac{7}{60}$ of the work in a day.

- To find how long they take to do 70% of the work, set up the equation: $\frac{7}{60}\times \text{days}=0.7$.

- Solving for the days needed gives: $\text{days}=\frac{0.7\times 60}{7}=6$.

- Therefore, they need 6 days to complete 70% of the work.

Correct answer is: Option:2, 6