Answer : Option C

Explanation :

A is twice as good as workman as B so he will take less time to do a piece of work than B and the ratio of time taken by A and B would be = 1:2

So, if there is a difference of one day (2-1), B takes 2 days.

As per question the difference in time taken by A and B is 40 days.

So, if the difference is of 40 days, B will take 2 * 40 = 80 days.

A takes 40 days less than B. So, A will take 40 days (80 - 40) to do the work.

A's one day work = 1
                              40

B's one day work = 1
                              80

(A + B)'s one day work = 1    ₊   1   ₌   120   ₌   3
                                      40        80      3200    80

So, working together they will do the work in 80  days
                                                                   3