What was the percent error of your answer?

How would your estimates have to change to get a more accurate answer?

Take a long piece of string. Draw a circle in chalk outside using that string as its radius. Estimate how many cars would fit inside then calculate it.

It's estimated that 10,000 different Hot Wheels cars have been produced since 1967. How large of a circle would it take to fit all those cars inside?

http://en.wikipedia.org/wiki/Hot_Wheels