12 men —> 9 days                           for 6km of road

1 man —-> 9 x 12 = 108 days        for 6km of road

36men —–> 108/36 = 3 days      for 6km of road

Now 48km of road = 8 times of 6km of road,

So 36 men —->  3 x 8 = 24 days for 48km of road.

Great answer!  Thanks.

Assumption: Each man contributes the same amount of effort regardless of the length of time 🙂