S = 16.9 +17.9 +18.9+ 19.9+ 20.9 怎么简算?
=18x5 + 0.9x5 = 90+4.5 = 94.5
因为：S = 16+17+18+19+20 + 5x0.9 = 5x18+5+0.9 = 94.5
=17+18+19+20+21-（0.1乘以5）=（17+21）乘以2.5-0.5=94.5
或者=16+17+18+19+20+（0.9乘以5）=（16+20）乘以2.5-+4.5=94.5
(17+18)+(19+20+21)-0.5=60+35-0.5=84.5
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