我想问一道英文的数学题!.我要过程John leaves home and start walking at 4 km/h to his friend's house. Half and a hour later, his sister Susan leaves the house on a bicycle and heads in the same direction at 12 km/h. How far away are they

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我想问一道英文的数学题!.我要过程
John leaves home and start walking at 4 km/h to his friend's house. Half and a hour later, his sister Susan leaves the house on a bicycle and heads in the same direction at 12 km/h. How far away are they from the house when Susan catches up to John, and how long after John fist left the house? 要用方程来解答
最后的fist应该是 first

约翰从家里离开以4km/h的速度去朋友家一个半小时之后,他的姐姐苏珊骑车沿同一路线出发,速度为12km/h.问当苏珊追上约翰时离家多远,此时约翰离家多长时间?
当姐姐出发时弟弟走的路程：4*1.5=6（km）
姐姐追上弟弟所需时间：6/（12-4）=0.75（h）（追及问题）
这段时间里姐姐走了0.75*12=9（km）即距家9km
所以弟弟走了9/4=2.25(h) (或1.5+0.75=2.5（h）)
答：——————————……

4*1.5=6km 6/（12-4）=0.75h 12*0.75=9km ∴离家9km 在john离开家0.75h后

设John离开家后x小时候Susan追上John,他们共行路程为s千米，由题可知
4x=12×(x-1,5)
=> x=9/4
s=4×9/4=9(km)
所以，John离开家2小时15分钟后Susan追上John，离家9千米。

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