A 2kg object submerged in the unknown fluid has an apparent loss of mass of 0.5kg. What is the specific gravity of the object?

The answer should be 20. The book's explanation is very confusing to me. Maybe someone can offer a better explanation? Also, how does the weight loss of the object has to do with the specific gravity of the unknown fluid?

Also, for #107, why is that the speed equals frequency times circumference of the golf ball? The question asks

The air very close to the surface of the ball is dragged along by a golf ball's spinning motion so that it moves at the same speed as the surface of the ball. If a golf ball in flight spins with a frequency of 60Hz, what is the approximate speed w of the air at its surface?

Many thanks!