Answer: 80 feet.
Step-by-step explanation: Let the width of the lot be x feet.
Given, the length of the lot is 120 feet.
Also, the perimeter of the lot is 400 feet.
Perimeter of a rectangle = 2 (Length + Width)
So, 400 = 2 (120 + x)
Simplifying the above equation, we get:
200 = 120 + x
x = 200 - 120
x = 80
Therefore, the width of the lot is 80 feet. ;D