9. You find a rock 7,500 km away from the nearest seafloor spreading center. The plate is spreading at 5 cm/year, how old is the rock? Give answer in years. (Hint: 100,000 cm = 1 km)

The answer is 150,000,000 years (150 million years)

Let's first convert 7,500 km to cm:

1 km = 100,000 cm

7,500 km = 7,500 · 1 km = 7,500 · 100,000 cm = 750,000,000 cm

Now, it is known that the plate is spreading at 5 cm per 1 year. Make a proportion:

5 cm : 1 year = 750,000,000 cm : x

x = 750,000,000 cm · 1 year : 5 cm = 150,000,000 years