Exercise 2 solution

2.5 cm a year is the total opening rate (half to each side). Over 10 million years:
2.5 cm x 10,000,000 = 25,000,000 cm = 250,000 m = 250 km.

The two points are therefore about 250 km farther apart. The Atlantic is thousands of kilometres wide (about 5,000 km at its narrowest between Brazil and West Africa, check), so the ocean took far longer than 10 million years to open: on the order of 100 million years or more, consistent with the age of the oldest Atlantic crust.