Correct Answer - Option 2 : 12

**Given:**

A and B working separately can do a piece of work in 10 days and 15 days respectively.

**Calculations:**

Time taken by A to complete the work = 10 days

so part of work done by A in 1 day = 1/10

similarly, part of work done by B in 1 day = 1/15

work done by (A + B) in 2 days = (1/10 + 1/15) = 1/6

Work done by (A + B) in one day = 1/12

Time taken by (A + B) to complete the work = 12 days

**∴ the correct answer is 12 days.**