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.