What is the scale of the map? for this activity, you will use centimeters. 1 cm = 405 km choose one strip of seafloor rock. what is the age of the rock? 20 use your ruler to measure the distance of the strip from the mid-atlantic ridge. 3 cm using the calculator, multiply the answer to #4 by 405 to figure out its distance in kilometers. round to the nearest whole number. i multiply 3 by 405 and get 1215 km determining rate of spreading use the age of the rock you have chosen and its distance from the mid-atlantic ridge to calculate seafloor spreading. first you will find out how fast the seafloor is moving away from the ridge on one side. use a calculator to divide the answer to #5 by the answer to #3. round to the nearest whole number. #5 km ÷ #3 million years = km/million years