Answer:
In this problem, we do not have the scale of the blueprint, but il try to answer it.
Suppose that the scale of the blueprint is X. This means that the things we saw in the blueprint are X times larger/smaller than the things that are represented. For example, if X = 1/10 this means that the dimensions in the blueprint are 10 times smaller than the real dimensions.
if X = 3 this means that the dimensions in the blueprint are 3 times larger than the real dimensions.
so if the width of the door is 4 feet, the width of the door in the blueprint will be: W = X*4ft
if the width of the mantel is 8 ft, the width in the blueprint will be W = X*8ft
Now you need to replace the value of X and solve those multiplications.
For example, if X = 1/2
we will have that the width of the door is W = (1/2)*4ft = 2ft
and the width of the mantel is W = (1/2)*8ft = 4ft