Every day, 100 students enter a school that has 100 lockers. All the lockers are closed when they arrive.
Student 1 opens every locker.
Student 2 closes every second locker.
Student 3 changes the state of every third locker i.e. he opens it if it is closed and closes it if its open.
Student 4 changes the state of every fourth locker and so on... so that student changes the state of every locker.
One day, however a few of the students are absent. Regardless, those present complete the procedure and simply skip the students who are absent. For e.g. if student 3 is absent, then nobody changes the state of every third locker.
At the end of the process, it is found that only locker number 1 is open and all the other 99 lockers are closed.
How many students were absent that day ?