Mental Math Tricks To Become A Human Calculator (For Speed Math, Math Tricks, Vedic Math Enthusiasts, GMAT, GRE, SAT Students & Case Interview Study Book 1)
 
http://www.techbuzon.com/2017/07/maths-in-minutes-easy-to-understand.html

ou don’t have to buy this book. Just read this till the end

You don’t have to buy this book. Just read this till end & you will learn something that will change the way you do math forever. Warning: I am revealing this secret only to the first set of readers who will buy this book & plan to put this secret back inside the book once I have enough sales. So read this until the very end while you still can.

School taught you the wrong way to do math

The way you were taught to do math, uses a lot of working memory. Working memory is the short term memory used to complete a mental task. You struggle because trying to do mental math the way you were taught in school, overloads your working memory. Let me show you what I mean with an example:
Try to multiply the 73201 x 3. To do this you multiply the following:
1 x 3 =
0 x 3 =
2 x 3 =
3 x 3 =
7 x 3 =
This wasn’t hard, & it might have taken you just seconds to multiply the individual numbers. However, to get the final answer, you need to remember every single digit you calculated to put them back together. It takes effort to get the answer because you spend time trying to recall the numbers you already calculated. Math would be easier to do in your head if you didn’t have to remember so many numbers. 
Imagine when you tried to multiply 73201 x 3, if you could have come up with the answer, in the time it took you to multiply the individual numbers. Wouldn’t you have solved the problem faster than the time it would have taken you to punch in the numbers inside a calculator?