我想问一道英文的数学题!John leaves home and start walking at 4 km/h to his friend's house.Half 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?

问题描述:

我想问一道英文的数学题!
John leaves home and start walking at 4 km/h to his friend's house.Half 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?

John离开家以4 km/h的速度到他朋友家去。半小时后,他的姐姐Susan骑自行车离开家并以12 km/h 的速度同一方向追赶他。当他们离家多远时,Susan追上John.并用时多少?

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