if 16 men can build a road of 9 km in 3 days then how many men can build 21 km road in 5 days.
we have
9km/(16men*3days) = 3/16 km/man-day
so, you conversion factor is
16/3 man-day/km
so, with 21km and 5 days, you will need
16/3 man-day/km * 21km / 5day = 22.4 men