There is 1000 doors (label 1,2,3...,999,1000) and 1000 peoples. First person close all doors, 2nd people open doors with label multiple of 2, 3rd person open doors with label multiple of 3 and so on. If the doors is open, he will close it. After 1000 people do that, how many doors are CLOSED? I has the answer, but I'm not understand it. Can someone explain how to do this simple math? Thanks...