A 0.25 kg rubber stopper attached to a 1.5 m string is moving in a circular path. The rubber stopper completes a lap in 0.4 seconds. What is the velocity of the rubber stopper?

It has a string of 1.5m, and completes a loop in 0.4s. So in 0.4s it does 2\times \pi\times1.5m. Then use distance=speed\timestime.