Let the length of the rectangular field be x.
Given, the width of the field is 3/8 of the length.
So width =
= 
Given, the width of the rectangular field = 240m
So we can write the equation as,

First we have to move 8 to the right side by multiplying 8 to both sides.



We can get x, by moving 3 to the right side by dividing 3 to both sides.



So we have got the length of the rectangular field = 640m = 0.64 km.
width of the field = 240m = 0.24 km.
We know the formula to find the area of the rectangle = 
where,
= length and
= width
By plugging in the values in the formula we will get,
Area = 

=
=
( Approximately taken to the nearest thousandth)
So we have got the required answer.
The area of the rectangular field = 