There are two islands on opposite sides of a mid-ocean ridge in the Atlantic Ocean. During the last 8 million years, the distance between the islands has increased by 200 kilometers. Calculate the rate at which the two plates are diverging
To calculate the rate (speed) of an object, you can use the formula "speed = distance/time."
The distance of the two islands has increased by 200 km. It took them 8 million years to do that.
200 km / 8,000,000 year = .000025 km/year
That number is next to incomprehensible, so it is best to convert the kilometers into a number that is much easier to visualize.
.000025 km/year is the same as 2.5 cm/year or 25 mm/year.
On average a human fingernail grows about 3 mm/month, which is equivalent to 36 mm/year. In other words, the two tectonic plates in your question are moving apart at a rate that is slightly slower than the rate your fingernails are growing right now. Which is why it took 8 million years to move only 200 km.