No Calculator? No Problem! Mastering Mental Math by Art Benjamin
2020, book 33: “(There is a) phonetic code for remembering longer sequences of numbers. Each number has an associated consonant sound — Art Benjamin
Finished on May 18, 2020
I’ve always enjoyed math and doing work mentally. I thought it would be cool and valuable to refine some of the things that I practice and use regularly. Working through this book was pretty cool. It was interesting to listen at the normal speed I do and strive to solve the math problems at the same time. I caught myself needing to slow down on many occasions so that I could properly assess myself and the things that I was studying.
Super interesting and quick trick that I’m sure most people are not aware of. This was one of the most applicable takeaways from this book.
When squaring a number that ends in 5, multiply the first number by the next number on the number line, and remember, it will always end in 25.
Example: 25x25 is 2x3=6 with 25 at the end=625
Example: 95x95 is 9x10=90 with 25 at the end=9025
I loved this particular piece of the book. While probably not incredibly applicable on the regular day to day, definitely interesting and valuable when needed.
To do 2 digit multiplication, take 17x15 and add the second digit of the second number to the first then multiply by 10. 17+5=22x10=220. Then add 7x5=35 to get to 255.
This works for numbers between 10 and 20.
When doing some quick calculations, this is certainly useful and helpful when you want to just get things done without using a calculator.
The close together method:
For use whenever the 2 digit numbers share the same first number.
For squaring numbers, you move to the nearest young number and go the same number of digits in the opposite direction also. Then add the square of the distance you moved.
Example: 49x49 is easier as 50x48 = 2400 + 1² = 2401
Example: 53x51 is easier as 54x50 = 2700 + 3x1 = 2703
I have yet to use this method fully but can see how it would be valuable — especially with things like phone numbers, bank account numbers, or anything similar.
Phonetic code for remembering longer sequences of numbers. Each number has an associated consonant sound:
1: T or D sound
2. N sound
3. M sound
4. R sound
5. L sound
6. Ch, Sh, or J sound
7. K or G sound
8. F or V sound
9. P or B sound
0. S or Z sound