Ans: 4 Men.
Explanation::::
3 men completes 1/8 th of work each day.
So 1 individual man can do (1/8)/3=(1/24) work per day.
Similarly, In order to complete the work in 6 days.
"x" no. of men need to complete 1/6th of work per day.
So,
1 man completes (1/6x)th work per day.
As the work power for each individual man i.e., portion of work completed per day by each man is same or equal. We can derive as
(1/24)th work/day/man= (1/6x)th work/day/man.
=> x= 4.
So 4 men can complete the same work in 6 days. Assuming each man has ideally same work power.