Tom ran around a ring track,which is 360 meters.During the first half of the whole time,his speed wa

题目

Tom ran around a ring track,which is 360 meters.During the first half of the whole time,his speed was 5 meters every second.During the second half,his speed was 4 meters every second.How many seconds did he spend on the second half of the whole way?

答案

速度乘以时间等于路程,总路程是360米,前半段时间的速度是每秒5米,后半段是每秒4米.由于Tom跑每秒5米和每秒4米都用了总时间的一半,设总时间的一半为x,故前段路程等于5x（速度乘以时间）,后段等于4x（注：5x不等于一半路程,4x也是）.两段路程合起来就是总路程,5x + 4x = 360.然后合并同类项和解方程,得x = 40.问题求的是后半段路程所用时间,路程的一半是180米.Tom最后用每秒4米的速度跑了4 × 40 = 160米,还有20米是用每秒5米的速度跑的,而这段一共是用了20 ÷ 5 = 4秒.这4秒加上后面换了速度跑的40秒,得出答案Tom一共用了44秒去跑后半段180米的路程.