Please give thorough explanation ty

Two objects have a perfectly elastic collision, as exactly illustrated in the diagram below. The first object (red ball) comes in at some initial speed and strikes the second one (blue ball), which was initially stationary in our reference frame. What is the speed of the blue ball after the collision? Write your answer as a fraction of v₁. In other words, if you get v₂ = 0.20v₁, just enter 0.20.

All we know is the proportionality of the masses and the relative speed of the first ball before and after the collision.