边长5cm=0.05m0.05X0.05X2.4=0.006立方米7.8X0.006=0.0468千克
5厘米=0.05米0.05*0.05=0.0025(平方米)0.0025*2.4=0.006(立方米)0.006*7.8=0.0468(千克)