How would you work out this question?

An island chain is 2100 km long and the ages of the rocks of the 2 end islands differ by 42 million years, what is the average rate of plate movement in cm yr-1

The average speed of an object is simply the distance travelled divided by the time. So here you divide 2100 km by 42 million years, and then convert the units to cm/yr. Work it out, then post back with the answer you get and we'll check it for you.

OK so:

2100 / 42 million= 0.00005

0.00005 converted = 5 cm yr-1

You got it.