The farm is rectangular, it has an area of 1/2 miles² and we know its length is 1/3mile.
To determine the width of the farm you have to use the formula of the Area of a rectangle.
The area of a rectangle is calculated by multiplying its length and width

From this formula you can calculate the width of the rectangle by dividing the area by the length:

For A= 1/2 and l=1/3

The width of the farm is 3/2 miles