【答案】：
45.74
2.3
21.7
解：(2)①：$58.7-24.4=34.3(\mathrm {g})$
②：$58.1-24.0=34.1(\mathrm {g})$
③：$55.2-13.0=42.2(\mathrm {g})$
④：$54.3-20=34.3(\mathrm {g})$
⑤：$55.8-21.7=34.1(\mathrm {g})$
所以③差距最大.
所以這枚錢幣實(shí)際質(zhì)量為$55.2-34.2=21.0(\mathrm {g})$
【解析】：
（1）45.74,2.3,21.7；
（2）設(shè)密封盒質(zhì)量為$x$g，古錢幣標(biāo)注質(zhì)量總和為$24.4 + 21.7 + 22.3 + 21.9 + 20.6 = 110.9$g，總質(zhì)量總和為$58.7 + 58.1 + 55.2 + 54.3 + 55.8 = 282.1$g，$5x = 282.1 - 110.9$，$x = 34.24$。①實(shí)際質(zhì)量：$58.7 - 34.24 = 24.46$g，②：$58.1 - 34.24 = 23.86$g，③：$55.2 - 34.24 = 20.96\approx21.0$g，④：$54.3 - 34.24 = 20.06$g，⑤：$55.8 - 34.24 = 21.56$g。③差異較大，實(shí)際質(zhì)量約21.0g。
