Work done by group 2 in a day = (1/180)th portion of the work
Now if they work together, the work done by both the group will be added,
i.e. total work done in 1 day by both group = [(1/240) + (1/180)] part of the work.
= [(3+4)/720] = (7/720)th part of work.
time required to complete the work = 720/7 days = 102.86 day.
well, that was the complete solution!