Explanation:
Speed has only magnitude.
Velocity has magnitude and direction.
"A bike goes 25 m/s toward main street."
This includes a direction, so it is velocity (V).
"A person walks 4 mph."
This has only magnitude, so it is speed (S).
"A plane flies 200 m/s."
This has only magnitude, so it is speed (S).
"A bird flies 100 mph due south."
This includes a direction, so it is velocity (V).