（长沙）A，B，C三个试管中各盛有10克、20克、30克水．把某种浓度的盐水10克倒入A中，混合后取出10克倒入B中，混合后又从B中取出 10克倒入C中．现在C中盐水浓度是0.5%．问最早倒入A中的盐水浓度是多少？
口算：× 45×÷10 0÷ 54×÷×64 12÷（﹣）×24 3×÷3×．
直接写出得数×= 1﹣=÷1=÷=×= 2.5×=×= 52×=×÷×= 0÷5×=
直接写得数．= 24÷====﹣==﹣×=
直接写出得数： 3﹣1.2=（0.1）3= 3÷1=×0.4=﹣=×7=+1= 5×÷5×=+×= 21÷3=
直接写得数．×4= 6﹣=÷=×=×=÷=÷=÷=
×
45×
÷10 0÷
54×
÷
×64 12÷
（
﹣
）×24 3×
÷3×
×
= 1﹣
=
÷1=
÷
=
×
= 2.5×
=
×
= 52×
=
×
÷
×
= 0÷5×
=
= 24÷
=
=
=
=
﹣
=
=
=
﹣1.2=（0.1）3= 3÷1
=
×0.4=
﹣
=
×7=
+1
= 5×
÷5×
+
×
= 21
÷3=
×4= 6﹣
=
÷
=
×
×
=
÷
=
÷
=
÷
=
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