When you combine two velocities going in opposite directions, you find the resultant velocity by subtracting the smaller velocity from the larger velocity.

Here's why:

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* When velocities are in opposite directions, they have opposite signs.

* Adding vectors with opposite signs is equivalent to subtraction.

Example:

* Object A is moving at 10 m/s to the right.

* Object B is moving at 5 m/s to the left.

To find the resultant velocity of Object A relative to Object B, we do:

* Resultant velocity = 10 m/s - (-5 m/s) = 15 m/s

Important Note: The direction of the resultant velocity is always in the direction of the larger velocity. In this case, the resultant velocity is 15 m/s to the right.