The number of times the window on the car is wider than a window on the model is nothing but the scale factor.
To find the scale factor, let us divide the original length of the car by its model's length.
Length of the original car = 10.5 feet = 10.5(12) = 126 inches.
Length of the model car = 7 inches.
So, scale factor = 126/7
= 18
Hence, a window on the car is 18 times wider than a window on the model.