It's been a while since I've done this, but I think it goes like this
We know the volume of the sphere formula is 4/3 pi r^3
So for the radius that we do know
4/3 pi (4)^3 it will be divided by 4/3 pi r^3
You can see from this 4/3 and pi will cancel each other out so you will only have
4^3/r^3 or 64/r^3.
You will set this equal to the ratios of the weight
64/r^3 = 5/35
We cross multiply
64 x 35 = 5 r^3. 64 x 35 = 2240. So you get 2240=5r^3. You will divide the 5 and cube root it to get the answer which is approximately 7.6517 cm.
Hope this helps!!