Pls explain Pigeonhole Principle

hi guys, i'm a dropper and in my previous attempt the biggest mistake i made was only studying arithmetic , so my arithmetic is kinda good but all the other topics are not that strong . so i have decided to start with algebra then modern maths and then arithmetic . is this strategy good pls let me know and pls suggest how do i make my foundations strong in algebra and what chapters should i start with..?

Is there any specific order in which I should watch the QA videos so I'm not confused.the topics are related to each other and I don't want to do a certain topic without completing the one I should've done before that.

I hope you understand what my problem is

Looking forward to your guidance

same as title

so in comprehensive example the question which is solved i aint under standing that even a single thing like its just not feeding into me so could anyone expalin it to me easily i am unable to ss the question its blacking out