Dilations involve multiplication, so we have that the dilation with scale factor 1/2 of the point (3,4) is (1.5, 2)
[tex]\frac{1}{2}\cdot(3,4)=(\frac{3}{2},\frac{4}{2})=(1.5,2)[/tex]and the dilation with scale factor 2 of the point (4,15) is (8, 30)
[tex]2\cdot(4,15)=(2\cdot4,2\cdot15)=(8,30)[/tex]