If the bob is fully immersed in the water, then the volume of water displaced will be equal to the volume of the bob = 4/3πr3, where r is the radius of the bob. This volume of water in the cylinder = πr2l, where r = radius of the cylinder, and l = level of water rose in the cylinder. Therefore, π x r2 x l = 4/3 x π x 33 Then r2 = 36 r = √36 r = 6cm