You have that the point (20,15) is a dilation of the point (4,3).
In order to determine the scale factor of dilation, you consider that when a point (x,y) is dilated by a factor k, the new point is given by:
(x,y) => (kx , ky) where (kx,ky) is the dilated point
that is, each new coordinate is:
x'= kx
y'= ky
You can solve for k in one of the previous equation, replace the values of the coordinates and obtain the value of k, just as follow:
k = x'/x = 20/4 = 5
k = y'/y = 15/3 = 5
Hence, the scale factor of dilation is k = 5
