A car with a mass M traveling to the East at speed v collides head-on with a truck of mass 2M, traveling to the west at speed v. If they stick and move together upon colliding, what is the initial velocity of the wreckage after the collision?

Answer 1

If east is + and west is -, then
Mv - 2Mv = (M-2M)v = -Mv = (M+2M) * -v/3
so, it will move west with speed v/3