Q.
Suppose that two objects A and B are moving with velocities VA and VB (each with respect to some common frame of reference). Let VAB represents the velocity of A with respect to B then:
Velocity of object A relative to that of object B is VAB=VA−VB
Velocity of object B relative to that of A is ∴VBA=VB−VA ∴VBA=−VAB
and ∣∣VAB∣∣=∣∣VBA∣∣