Hi Jyotishman,
as for structuring, the best preparation I found for math-intensive case has been on consulting handbooks of top MBA school. In particular, I would suggest the following:
As for math speed and accuracy, you can find several resources online where you can test yourself under pressure. PrepLounge and Victor Cheng websites in particular have an ad-hoc section for that; you can check them at the following links:
https://www.caseinterview.com/math/home.php
https://www.preplounge.com/en/mental-math.php
Hope this helps,
Francesco
Dear Jyotishman,
a good way is to practice with GMAT math exercises. They might of course be a bit different, but they train you at being quick in problemsolving.
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Dear Jyotishman!
My advice to you is to practice fast math: learn the division table up to 1/11, learn how to multiply double-digit numbers, how to work with zeros. You can actually use math tools (Mimir math for iOS), Math tool on Viktor Cheng website to practice.
To practice your quantitative skills I would recommend different math, logic, and verbal tests on https://trytalentq.com/how-to-prepare/# and then select "Elements Practice Test".
You don´t need to learn them by heart, rather skipping through to get a first impression and a better understanding.
Hope it helps,
Best,
André