There is a YouTube channel by the name wifi study channel. Go to that channel and watch videos on general math. It's an Indian channel. Videos of a person named sahil, are available on math. It will cover your whole ability portion of GSA. The guy has a really good method of teaching. It helped me a lot.
Just make sure you go in the right section of this channel, that is RRB GROUP D 2018, math by Sahil sir. Because he also teaches advance math for some engineering exams on that channel.
He uses a board, explains every mathematical step of a question and explains it well. Also, you need to start watching from the very 1st video. Videos are numbered and go on systematically. If you miss even one class/video of the basics, you won't get the next ones.
P.s i don't know how to start a new thread. If someone here does, please post this in a separate thread to help all the aspirants. I know that a hell lot of aspirants need help with this portion and there are no online classes, groups available on fb or anywhere else for this portion.

Can you please further specify the most relevant videos?The section you mentioned (RRB Group D 2018/Maths by Sahil sir ) contains some 353 videos.


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Videos are numbered. You have to start watching from the video number 1. In the very 1st video, the guy gives the breakup of the whole course/syllabus, how he'll be covering it by dividing it into 3 sections. The 1st 11 videos are only on BODMAS, fractions and decimals with a test. I had no idea of the basic concepts of fractions so I started from the video number 1.
Also let me clarify that these 1st 11 videos do not deal with the theoretical questions as they appear in css. But with basic concepts which you'll be needing to decode those theoretical questions. So, it might take 1st 50-60 videos to cover all the content. I haven't reached the end yet. So, you need to start from the video # 1. See the breakup of the course given in the very 1st video and decide for yourself which section you need help with. From that breakup you'll be better able to gauge that you need to start watching from what video. If the 1st 12-15 videos are only on section 1( BODMAS, fractions, decimals, square root) and you don't need help with that, you can skip those and start from video number 15 on wards, where he covers number system, LCM, HCF, ratio, proportion, %age and so on.. watch the video number 1, you'll have a better idea.