Calculating percentages are important as they apply to real life activities. For example, shopping. If you are on a budget and you want to know how much your total is with tax, you can convert the decimal for the tax, turn it into a percent and add it to your total. Another example is eating at a restaurant. A good tip is 15-20% so you can calculate this and add it to your price.
The surface area of a sphere is where r is the radius; The circumference is so that using this value we can evaluate the surface area of 1 ball as: For 100 ball you get:
where r is the radius;
so that 
using this value we can evaluate the surface area of 1 ball as:
