一艘轮船从甲港开往乙港每小时30千米，返回时逆水，每小时20千米，求往返的平均速度．
计算下面各题．．
×÷÷÷×÷÷．
直接写出得数．×=××=÷3= 12×=+﹣+= 20÷= 18÷÷=×= 0×=×÷×=
口算 56×= 2﹣=+= 6×= 0.25×4= 57×=÷= 1÷= 21÷=÷= 1÷= 0÷=
计算下面各题．×21×÷5×÷÷÷÷24×÷8．
．
×
÷
÷
÷
×
÷
÷
．
×
=
×
×
=
÷3= 12×
=
+
﹣
+
=
= 18÷
÷
=
= 0×
=
×
÷
= 2﹣
=
+
= 6×
= 0.25×4= 57×
=
÷
= 1÷
= 21÷
=
÷
= 1÷
= 0÷
=
×21×
÷5×
÷
÷
÷
÷24
×
÷8．
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