As CaptainBlack has pointed out, each man does -th of the work each day.
For 5 days, we have 120 men doing work.
-th of the work has been done in those five days.
Now we have 90 men working. Let's assign a variable x to the number of days required to finish the work.
of the work is done each day with 90 men.
now we have:
because 1/40 of the work was done in the first five days.
x=260 days.
Now we add the initial 5 days and we come to a total of 265 days.