One hundred students are assigned lockers 1 through 100. The students assigned to locker 1 opens all 100 lockers. The student assigned to locker 2 closes all lockers that are multiples of 2. The student assigned to locker 3 changes the status of all lockers ( e.g., locker 3 is open gets closed and locker 6 that is closed gets opened) The student assigned to locker 4 changed the status of all lockers whose multiples of 4, and so on for all 100 lockers.

PLEASE HELP ME I HAVEN'T STARTED ITS DUE TMRW

Identifying which students touch which lockers is a little less of a brute-force approach and would likely have gotten you to the solution a little more quickly.

Here’s what I mean:

Consider locker #1. The only student who touches locker #1 is student #1. Student 1 opens the locker, and since no one else touches it, it will be left open at the end.

Consider locker #2. Student 1 opens the locker, and student 2 closes it. No one else touches the locker, so it will be closed.

Consider locker #10. Students 1 opens the locker. Student 2 closes it. Students 3 and 4 skip right by it. Student 5 opens it. Students 6, 7, 8, and 9 skip right by it. And student 10 closes it. Locker #10 will be closed.

Mental Milestone 1: After looking at several lockers, you should notice that lockers are only changed by student numbers that are factors of the locker number. In other words, locker 12 is changed by students 1, 2, 3, 4, 6, and 12.

Mental Milestone 2: You should also have noticed that factors always come in pairs. This means that for every student who opens a locker, there is another student who closes it. For locker #12, student 1 opens it, but student 12 closes it later. Student 2 opens it, but student 6 closes it later. Student 3 opens it, but student 4 closes it later.

By this logic, every locker would be closed.

But there are exceptions!

Consider locker #25. Student 1 opens it. Student 5 closes it. Student 25 opens it. The locker will be left open, but why? In this case, the factors do not come in pairs. One and 25 are a pair, but five times five is also 25. Five only counts as one factor. This causes the open-close pattern to be thrown off. Locker #25 is left open.

Mental Milestone 3: When factors don’t come in pairs, the locker will be left open. And factors don’t come in pairs when numbers are multiplied by themselves. Perfect squares (1, 4, 9, 16, 25, 36, 49, 64, 81, 100,…) are the only numbers whose factors don’t come in pairs because one set of factors, the square root, is multiplied by itself. This means that only perfect square lockers will be left open.

Locker #s 1, 4, 9, 16, 25, 36, 49, 64, 81, and 100 are left open!