Do you know the length and height of the field? If you do, all all the sides together (Height + Height + Length + Length) and you get the answer to Part A.
Part B:
Just do 120 times 5. Then, divide your answer by 6. The answer will be the time they need to run each lap in.
Part C:
Again, what is the length? When you know your length, divide it by 2. Your answer is the side length.
Hope that helped. :)