Bhal's Library Quiz
In a school library, there are 1000 boxes which are numbered from 1 to 1000. 1000 students are selected from school and they are also numbered from 1 to 1000. Initially all the boxes are open. Student no. n goes and change the configuration of every box whose number is divisible by the student's number. This process is done by all 1000 students one by one from 1 to 1000. Finally how many boxes are open?