Today the Atlantic Ocean is about 5,700 km wide at the latitude of Boston. At one time, however, there was no Atlantic Ocean because the east coast of the United States and the northwest coast of Africa were joined in a huge supercontinent. The Atlantic Ocean started to form "only" 185 million years ago, as modern North America split from Africa and the two continents slowly drifted apart in a process called seafloor spreading. Assuming that the rate of seafloor spreading has been constant, at what rate has North America been moving away from Africa? Give your answer in millimeters and kilometers.