Explanation:

12- men do a work in =8 days.

In 3- days 12- men can do= 3/8 the work.

In 1-day 1- man can do= 1/12 x1/8 =1/96 the work.

So (12+3)= 15 men can do the remaining work in = 5/8 ÷ (15/96) =4days.

So the remaining 5/8 of work will be finished in 4- days.