Triangle ABC has vertices A(5, 3) , B(3, 7) , and C(1, 4) . Triangle ABC is dilated, and the vertices of the image are A′(7.5, 4.5) , B′(4.5, 10.5) , and C′(1.5, 6) . The center of dilation is the origin.

What is the scale factor of the dilation?