If A and B persons are moving with VA and VB velocities in opposite directions. Magnitude of relative velocity of B w.r.t. A is x and magnitude of relative velocity of A w.r.t B is y. Then

Suppose A is moving along positive x axis, so B must be moving along negative x-axis ∴ A→ =VA i^ and B→ =VB -i^∴ VAB→= VA→-VB→ = VA i^ -VB -i^=VA+VBi^and ∴ VBA→= VB→-VA→ =- VB i^ -VAi^ =-VA+VBi^VAB→=VBA→∴ x=y