"8 workers can plough a 6 ha field in 5 hours" therefore (8)(5) = 40 man-hours are required to plough a 6 ha field therefore 40/6 man-hours are required to plough a 1 ha field.

Therefore a 9 ha field requires 9 (40/6) = 60 man-hours. 60 man-hours divided by 5 workers = 12 hours.

So it takes 12 hours for 5 workers to plough a 9 ha field.

If you want to be PC you can replace man-hours with person-hours.