The mass of April and her bumper car is 80 kg
Explanation:
This problem can be solved by using the law of conservation of momentum.
In fact, the total momentum of the system must be conserved before and after the collision, so we have:
where:
is the mass of Fiona+her bumper
is the initial velocity of Fiona (we take her direction as positive direction)
is the final velocity of Fiona (she rebounds back)
is the mass of April+her bumper
is the initial velocity of April (in the opposite direction)
is the final velocity of April
Re-arranging the equation, we can find the mass of April+bumper:

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