一条大河，河中间（主航道）水速为每小时8千米，沿岸边水速为每小时6千米，一条船在河中间顺流而下，13小时行驶520千米，求这条船沿岸边返回原出发地点，需要多少小时？
直接写得数． 1÷=÷2= 4×20%=×4= （+）×12= 1÷75%= 1%÷10%= 42%﹣0.42=
直接写得数． 12×= 6÷=÷= 1÷×=×18=×0×=×=×26=
直接写出得数．×60= 12÷= 3.14×42=（）2=×=+=÷=÷=
直接写得数．×4=+=+=÷= 25÷=﹣=÷5=×= 2÷=﹣=×12= 2﹣﹣=
口算========
=
÷2= 4×20%=
×4=
+
）×12= 1÷75%= 1%÷10%= 42%﹣0.42=
= 6÷
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÷
= 1÷
×
=
×18=
×0×
=
×
=
×26=
×60= 12÷
= 3.14×42=（
）2=
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