One minute before 12:00, you place 10 balls labeled from 1 to 10 in the box. You randomly pick one ball out. In 1/2 min before 12:00, you put 10 ball in the box, and label them from 11 to 20. You then randomly pick one ball out. You continue in the manner until you reach 12:00. What is the number of balls in the box at 12:00?

In other worlds, for ( 1/2)^(n-1) before 12:00, we put and additional 10 balls in the box, and randomly pick one ball out. The question is how many balls in the box at 12:00 ?