Rate is the distance traveled by the speed or how fast. One could therefore say that, m/ph or km/h or miles/distance. In this case, 30 metres = 3000 cm in 1000 years. This means that you will divide 3000/1000 years to get 3 cm per year.
One meter is 100 cm which implies 30 meters is 30*100 cm which is 3000 cm. So the continent has moved 3000 cm in 1000 years. So the distance traveled in one year will be 3000 cm divided by 1000 that is 3000/1000 which is 3cm/year So the rate at which a continent is traveling is 3 cm/year.
