1000 Doors Homework Problem I am faced with the following problem as homework- a man has 1000 doors. he opens every door, and then he closes every second door. Then he works on every third door- if it's open, then he closes it. if it's  closed, he opens it. Then he works on every fourth door, fifth door, and so on all the way until the 1000th door. I need find the amount of doors left open and their numbers (Door 1, Door 2, etc.).
Can someone please tell me the first step to solving this and what kind of mathematics I need to use. 

This is an  absolute classic, in the first round he opens all of them, then he closes multiples of 2. Then he alters multiples of 3. So in round $j$ the door $a$ is opened or closed if and only if j divides it. How many times is each door altered think about its divisors.
full solution:

 a door $j$ is altered the same number of times as the number of divisors it has, so the open doors are the ones with an odd number of divisors, let $d$ divide $j$, then $j=dk$. So every divisor corresponds to another unique  divisor.  Unless we have $d=k$ in which case $j=d^2$. so the ones open are exactly the  perfect squares. Also  see: light bulb teaser

