However the question clearly says that the final velocity is in opposite direction to the initial velocity. This implies that we have (-v), final velocity in opposite direction.
We must recall that vectors in opposite direction are commonly shown using a negative sign. Velocity is a vector, hence it possesses both magnitude and direction as we can see from the question.
The answer I believe is 5 because 2 vases hold 10 red roses and 5 yellow roses. But since the person has 3 vases but it has to be the same amount as the first person. They would both have 30 flowers in their vases. But then each of the vases has 6 red roses so you just do 30 divided by 6 and you get 5.