问题标题：
我想问一道英文的数学题!Johnleaveshomeandstartwalkingat4km/htohisfriend'shouse.Halfahourlater,hissisterSusanleavesthehouseonabicycleandheadsinthesamedirectionat12km/h.Howfarawayaretheyfromthehousewhen
问题描述：
我想问一道英文的数学题!
Johnleaveshomeandstartwalkingat4km/htohisfriend'shouse.Halfahourlater,hissisterSusanleavesthehouseonabicycleandheadsinthesamedirectionat12km/h.HowfarawayaretheyfromthehousewhenSusancatchesuptoJohn,andhowlongafterJohnfistleftthehouse?
宋爱民回答：
Susan追John时,John已走了：4*0.5=2km
Susan追上John需要时间：2/(12-4)=0.25h
此时John离家距离：4*(0.5+0.25)=3km
John离开家时间为：0.5+0.25=0.75h=45分钟
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