This app was mentioned in 14 comments, with an average of 1.50 upvotes
download an app called Math Tricks and learn how to change your thinking on this to a 'can do'. Also check out youtube for learning math tricks. It'll take practice and dedication, like anything else, to learn it but once you do, understand that writing it down is not the same as thinking about it.
App: https://play.google.com/store/apps/details?id=example.matharithmetics&hl=en_US
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There is an app called Math Tricks on Android (might be on Apple too, I don't know) that is pretty useful, doesn't help with complex maths but is brilliant for reinforcing basic mental maths
This app will answer your question
Just in case you don't find it eventually, there's this other app called Math Tricks for Android which is pretty good too.
https://play.google.com/store/apps/details?id=example.matharithmetics&hl=de
xD need some competition on the leaderboard
Is it Math Tricks? But it only has math problems no computer science problems.
Maths is totally not appropriate for mnemonics in my experience and the experience of those I speak with. Sorry, dude.
Videos I can help with. Vihart has a wealth of great videos, so it's worth having a look at the back-catalogue. I presume you'll know Numberphile as well. Then there are umpteen million lectures. This link was for my subject, but whatever you're doing at the time there's a lecture on it.
Then there's the apps. My favourite is Maths Tricks as it's a fun and simple game, but it also explains new tricks for arithmetic (okay, so not actually Maths).
When it comes to these things, I often find motivation's lacking because academia doesn't provide gumption tools, so you have to make your own. Prisig talks a lot about this in Zen and the Art of Motorcycle Maintenance. There's the basics - good lighting, eat well, et c. - but for Maths specifically you can create new gumption tools, such as using the Maths you are learning. And you can always use it somehow, even if it's just for imaginary examples. Just find something to apply it to. If it's stats, get one of the free SPSS substitutes and have a play. If it's set theory, imagine programming a computer with sets rather than nand gates and see how far you get.
Upped tricks are basic ones, lul, look at that app: https://play.google.com/store/apps/details?id=example.matharithmetics