The scale factor is found by dividing the length of a side of the image by the length of the corresponding side on the original.
30/40 = 3/4
The scale factor is 3/4.
Triangle ABC is similar to triangle YZX.
YX/AB = 37.5/50 = 3/4
YZ/AB = 30/40 = 3/4
ZX/BC = 45/60 = 3/4
Each side of triangle YZX corresponds to a side of triangle ABC as shown above in the three scale factor segments.