Given:
A scale model of a building having the dimensions of 3 inches in length and 4 inches in width.
Here, 1 inch = 40 feet.
To find the actual dimensions of the base of the building:
Since 1 inch = 40 feet,
So we have,

Thus, the length of the base of the building is 120 feet and the width of the base of the building is 160 feet.