WILL CLINTON
   PARDON YOU???
 

500 crooks were locked in separate cells in a prison row. Before he left office, President Clinton use the following method to decide who he would pardon. First he unlocked every cell in the prison row. Next he locked every second cell. Then he turned the key in every third cell, locking those cells which were open and opening those cells which were locked. He continued this way, on the nth trip turning the key in every nth cell. Those crooks whose cells eventually remained open were pardoned. Who were the lucky ones?

Solution