Since some people are having trouble with night 6, I'm making this post with some tips to tell when an animatronic is gone and some other things.

Firstly, you don't need to shine your light or lift up the curtain until you hear the "RAAAAH" sound effect. When you hear it, you use it.

Secondly, with the character on the right you can tell when he's gone because when he's there there is a bit more light to the right (first two images in this post).

Then, for the one on the left, if you lift up the curtain you can see a bit of white from the jpeg through the door.

For the mimic there's no way to tell except if the character on the left is with him. There will be less white visible when that is the case.

For all 20 I don't think that it is possible to beat but I think these tips will help