I.E. Irodov Solution Problem 1.5 - A simple solution

Q.1.5 Two particles, 1 and 2, move with a constant velocity v1 and v2. At the initial moment their radius vectors are equal to r1 and r2. How must be these four vectors be interrelated for the particles to collide?

The solution uses concept of relative velocity, and the understanding of when a moving particle collides with a stationary particle.