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

Your answer is b. codominant