The distance around a circle is (2 · π · radius) .
Length of one revolution = (2 · π · 25 cm)
One revolution = 157.1 cm
One revolution = 1.571 meter
Six revolutions = (6 · 1.571 meter) = 9.425 meters
Speed = (distance covered) / (time to cover the distance)
Speed = (9.425 meters) / (4.0 sec)
Speed = 2.36 m/s