为挖通300米长的隧道，甲、乙两个施工队分别从隧道两端同时相对施工．第一天甲、乙两队各掘进了10米，从第二天起，甲队每天的工作效率总是前一天的2倍，乙队每天的工作效率总是前一天的倍．那么，两队挖通这条隧道需要多少天?
计算下面各题．×21×÷5×÷÷÷÷24×÷8．
计算．×9=×0= 0÷=÷= 12×=×=÷=÷=
直接写得数×=÷5=×12=×=÷=÷=×= 1÷== 45×=÷2=÷=×=÷=×0=×= 1﹣=÷=×= 15÷=
口算×6=+=×=×6=÷=÷= 1﹣=﹣=×4=÷=
直接写得数．×= 0×=÷=÷4= 12÷= 0.3×=×÷×= 0×+=
倍．那么，两队挖通这条隧道需要多少天?倍．那么，两队挖通这条隧道需要多少天?
×21×
÷5×
÷
÷
÷
÷24
×
÷8．
×9=
×0= 0÷
=
÷
=
×
=
÷
=
÷
=
×
=
÷5=
×12=
×
=
÷
÷
=
×
= 1÷
=
= 45×
=
÷2=
÷
=
×
=
=
×0=
×
= 1﹣
=
÷
=
×
= 15÷
=
×6=
+
=
×
=
×6=
÷
=
÷
= 1﹣
=
﹣
=
×4=
÷
=
×
= 0×
=
÷
=
÷4= 12÷
= 0.3×
=
×
÷
×
= 0×
+
=
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