I can not add a comment to the answer of hexomino because I have not enough reputation, but we can solve it also with the following equation:
(Being X half day of one equal group of workers)
3X = 2(X+1+1)
Which is read: "3 half days of an X group finish the work in the large field in the double of time that an X group plus 1 worker half day, and another worker another half of day (which is one worker the whole day) would finish the small field"
So if we solve the equation
[link to solver]:
3X = 2(X+1+1)
3X = 2(X+2)
3X = 2X + 4
3X - 2X = 4
X = 4
We find that half group of workers is composed by 4, so the whole group is composed by 8 workers.
And now we can check the numbers:
The large field is completed by 12 half day workers in total. So 4 workers manage to complete one third of the large field every half day. As the small field is half the size, one third there, is the double (2 thirds), so the group of 4 workers can manage to finish 2 thirds of the small field the rest of the half day, meaning that for every third, we need two workeres during half day, and that's why the last third is finished by one worker, working all day.