Resources for mastering mental math

First of all,I would like to apologise if this topic is redundant. Recently I found out about mental math. I started to explore it and I feel like I am losing a lot of time on finding resources ( for example sites; books; techinques,etc.). I was wondering if anyone can explain me the basic systems ( or give me the links to find out more ).

Thank you in advance; John

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Anything specific you are looking for? Addition, multiplication, roots, calendar, etc…

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