ρ铁=7.9×10^3㎏/m^3 = 7.9 (g/ml)
铁球体积 V球 = V总 - V水 = 75 - 60 = 15 ml
铁球密度 ρ球 = m球/V球 = 79 /15 (g/ml) < ρ铁
所以铁球空心
铁球实心体积
V实 = m球/ρ铁 = 79 / 7.9 = 10 ml
则空心体积
V空 = V球 - V实 = 15 -10 = 5 ml
(备注 :ml 毫升 = cm^3 立方厘米)