If each of 6 tractors ploughed 120 hectares a day and they finished the work would be finished in 4 days, then the whole field is:
120 x 6 x 4= 720
720 x 4=2880 hectares. Let's suppose that each of the four tractors ploughed x hectares a day. Therefore in 5 days they ploughed
5 x 4 x x=20 x x hectares, which equals the area of the whole field, 2880 hectares.
So, we get 20x=2880
x=2880 over 20 =144
. Hence, each of the four tractors would plough 144 hectares a day.