Answer:
3 huts are built by 5 people in 21 days.
So 1 hut is built by 5 people in 7 days.
That is to say, 1 hut requires 5*7 = 35 man-days.
Which means 6 huts need 6*35 = 210 man-days.
If the time available is 15 days, 6 huts will need 210 man-days/15 days = 14 people.
Answer = 14 people.