Consider a proton moving in 3 dimensions, whose motion is confined to be within a nucleus (a sphere of radius 2e-15 m) and whose momentum must have magnitude less than p0= 3e-19 kgm/s. Roughly how many quantum states are available to this proton? (Hint: The volume of a sphere of radius p0 is (4/3)pi*p30). According to the book, the answer is 13, I'm just not sure how they got that answer.