设车的速度是v,相邻两车间隔t分钟
(vt)/(v-v1)=10 (1)
vt/(v-3v1)=20 (2)
(2)/(1)
得(v-v1)/(v-3v1)=2
v=5v1
所以5v1t/(5v1+v1)=10
5t/6=10
t=12分钟