The radius of one sphere is twice as great as the radius of a second sphere. find the ratio of their volumes.

The ratio of the radii is 2:1, so the ratio of their surface areas is 4:1. The area of a sphere is defined by 4 * pi * r^2 

So if the radius of the first sphere is 2r, then it would be (2r)^2 = 4r^2 

The rest is the same. So the ratio of their surface areas would be 

4 * pi * 4r^2 divided by 
4 * pi * r^2 

or 4.