To find the area, the equation is A = pie x radius to the second power. So first divide the diameter by two to find the radius. Then multiply the radius to the second power and then multiply that by the pie. Once you get the area for both pizzas subtract them to see how much bigger the larger pizza is to the other one.
Stratification tells us that the closer something is to the surface, the newer it is. That means that artifacts found deep in the site are older. This helps to provide relative dating for artifacts.