把一根钢管锯成5段，锯断一次的时间是2.5分钟，锯成5段共用多少分钟？
计算下面各题．===﹣= 1﹣=﹣==﹣==
看谁算得又对又快．======．
3与4的倒数之差是多少？
算一算．+=﹣=+=+=﹣=﹣=
+= 1﹣=+=+= 1﹣=+=﹣=﹣=
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