设货车,客车,小轿车速度为v1,v2,v3,间距为d,则：10(v3-v1)=d,15(v3-v2)=2d求d/(v1-v2)
v3-v1=d/10,v3-v2=2d/15
v1-v2=2d/15-d/10,d/(v1-v2)=30
30-15=15min