A highly elastic ball is thrown at a vehicle close by, rank the resulting speed

Question

A highly elastic ball is thrown at a vehicle close by, rank the resulting speed of the ball from smallest to largest:
a.truck is stopped
b.truck moving towards you at 20 m/s
c.truck moving away from you at 20 m/s
d.truck moving towards you at 50 m/s
e.truck moving away from you at 50 m/s
Please explain how you got your answer. I.e. formula and such, not interested in a random guess. I know for a one dimensional perfectly elastic collision between two objects this is true: v1i + v1f = v2i + v2f not having any luck though.

Explanation / Answer

momentum, must be conserved, so, final momentum of the combination of the two bodies(truck + ball) must remain same, so, the smallest speed of the ball will result due to the highest speed of the truck, so, the order of ranking is : e.truck moving away from you at 50 m/s c.truck moving away from you at 20 m/s a.truck is stopped b.truck moving towards you at 20 m/s d.truck moving towards you at 50 m/s

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