Verified answer

The ball's circumference is 109mm / 13 (do you see why?)

109 mm / 13 = 8.3846 mm

The circumference of a sphere is equal to twice the radius times pi, so, if we call the radius r,

8.3846 = r X 2 X pi

8.3846 / (2 X pi) = r

8.3846 / (2 X 3.1416) = r

1.33445 = r

The radius of the ballis 1.33445 mm.

one kilogram = 2.205 pounds

one meter = 3.281 feet

one cubic meter = 3.281^3 cubic feet, or 35.32 cubic feet

so 1 kg/m^3 = 2.205lbs / 35.32 cubic feet, or 0.0624 lb/ft^3

so 28.6 lb/ft^3 = 38.6 / 0.0624 kg/m^3, or 618.3 kg/m^3