Suppose that X is a vertical segment that lies on the y-axis with its beginning at origin and ending at the point (0,4). Then the length of this segment is 4 units.
If you vertically compress this segment by factor of 1/4 with the centre of compression at the origin, you recieve a segment that also lies on the y-axis with its beginning at origin and ending at the point (0,1) (the length of this image segment is 1 unit).
So, the question is: if a segment that lies on the y-axis with its beginning at origin and ending at the point (0,4) is vertically compressed by factor of 1/4 with the centre of compression at the origin, what is the image of this transformation?