我想问一道英文的数学题!
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?
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