设x小时能追上慢车此时慢车行驶了100*x+100快车行驶销昌逗了140x此时快车比慢车多行亏卖驶400所迅态以有140x=100*x+100+40040x=500x=12.5小时
100/(140-100) = 100/40 =2.5 小时