请教一题

cpcjustin

A boy walking along a road at 3 kilometers per hour is overtaken by a truck traveling at 40 kilometers per hour. If the truck breaks down 1 kilometer beyond where it passes the boy, how many minutes after the breakdown does the boy reach the truck?

A  21.5      B 20       C  18.92      D 18.60       E  18.5

so easy problem, why QIAN YONG QIANG chose  E?

mafaldababy

E is right. the trunk took 1/40 hr to cover 1km. during this period, the boy covered 3/40 km. after breakdown, the distance which was covered by the boy is 1-3/40 km.   [(1-3/40)/3]*60=18.5 

cpcjustin

not understand yet. in elementary school, this promble is 1/3*60=20. why?

"during this period, the boy covered 3/40 km. after breakdown, the distance which was covered by the boy is 1-3/40 km. "

what do you mean?

bigdreamer

这题E是对的.当卡车超过行人用1.5分种行驶1公里.行人用20分走1公里.但卡车行驶这1公里时,行人并未停止,而是继续向前走,当卡车走完1公里,行人也走了1.5分钟.所以,最后的时间为20-1.5=18.5