When the two vehicles collide, then vehicle with slow speed will pushes back.
Speed of any moving body is define as the distance travelled by that body in per unit of time.
As it is mentioned that one car has a speed of 30 mph and another one has a speed of 60mph, if both of these cars get collide then the car with high speed pushed back the car with low speed.
Hence the car with slow speed pushed back.
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