Thought experiment #1 : you have infinitely many ping-pong balls, each labeled with a natural number and lined up in order 1,2,3.. in 30 seconds you put the first 10 ping pong balls in a barrel and then remove ball 1 in the next 15 seconds you put the next 2 balls in the barrel and then remove ball 2 in the next 7.5 seconds you put the next 10 balls in the barrel and then remove ball 3.. After this process completes ( 1 full minute passes) how many balls are left in the barrel?