How can I personally calculate the number of blocks needed for my building?
A building has 100 doors numbered from 1 to 100. A person opened all of the doors, and next he closes every 2nd door (starting 2, 4). When he finishes all 100 doors, he opens/closes every 3rd door (3, 6), then every 4th door and so on till the 100th. Which doors will be closed in the end?