Which of the following terms are needed to describe velocity?
number
direction
mass
or units

By definition, velocity is the rate of change of distance with respect to time.
Unlike speed, which is a scalar quantity, velocity is a vector that has both magnitude and direction.
Therefore velocity is speed, qualified by its direction.

Let us examine the statements given.
a. Number (Needed)
    The number provides the magnitude of velocity.

b. Direction (Needed)
    The direction is needed in order to define velocity as a vector.

c. Mass (Not needed)
    Mass is not required to define velocity.

d. Units (Needed)
   The magnitude of velocity is distance divided by time.
   Because distance has units (such as feet, meters or kilometers) and time
   has units (such as seconds, hours) velocity has units such as mile/hour,
   km/hour, ft/s and so on.

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