To compute the rate of spread, we must first take into account the constant values.
Rate of Spread = 3 centimeter per year
Now, we multiply 3 centimeters to 1 million years. Or,
Rate = 3 centimeter x 1,000,000 years
= 3,000,000 centimeters
Now, we find the kilometer equivalent for 3 million centimeters.
3,000,000 centimeters = 30 kilometers
Thus, after 1 million years, the Atlantic Ocean will spread at least 30 kilometers.