Mental Math Tricks

So how good are your mental math skills, ATS? Up until recently, mine have always been pretty useless. I admire Math, but it's scary. In an effort to increase my dream recall I happened upon this brain boosting book called:

"The secrets of Mental Math: The Mathemagicians Guide to Lightning Calculation and amazing math tricks"
www.amazon.com...

Now, I'm not quite the math genius yet, but I wanted to share this because the book is a pretty fun challenge and you learn some cool mental math tricks.
The first trick I learned in the book was this: If you want to find the square of any 2 digit number that ends in 5, all you do is multiply the first number by the next higher number and add 25 at the end: The square of 35 would be - (3*4) and attach 25 = 1225. That's it! Easy money.

If anybody knows some other tricks or just wants to brag about their mental math prowess, feel free to post.

Im smart enough not to say half pint. I just say cup.

a reply to: Tucket

I possess no special math prowess. However, sometime during my years I learned a quaint little arithmetic trick which I will present to you now.

Here goes:

1) Multiply any number by 9.

2) Obtain the digital root by adding each digit of the resulting product together. If this yields a sum comprised of two or more digits, repeat the process until you're left with a single digit.

3) It ALWAYS equals 9.

Examples:

A) 5×9=45; 4+5=9
B) 10×9=90; 9+0=9
C) 123,456,789×9=1,111,111,101; 1+1+1+1+1+1+1+1+1=9
D) 987,654,321×9=8,888,888,889; 8+8+8+8+8+8+8+8+8+9=81; 8+1=9

Try for yourself.

Please I beg of you someone explain casting out nines to me.

My third grade teacher showed us that been wondering about it now for 61 years

originally posted by: GBP/JPY
Please I beg of you someone explain it casting out nines to me.

Down demon, begone now I say! I cast thee back into the eternal pit of darkness from whence you came!

When I need the value of pi out to several decimal places and I only have a rudimentary calculator, I divide 113 into 355. Works close enough for me...

a reply to: DictionaryOfExcuses

Even better!!

Use your hands!!

Hold out both hands, backsides up.

1x9??

Drop your left pinky by curling to your palm!

2x9?

Same with the ring finger. Count your pinky as “10” and the ring finger as the gap. The rest of the fingers are the “ones” position of the result.

Walk your fingers through it! (Good hand calisthenics!!) [Each finger, left to right, one at a time...]

9, 18, 27, 36, 45, 54, 63, 72, 81!! (ETA: Forgot 90!!)

How cool is that??

9x6 and 9x5 always screwed me up until I learned this song be!



PS - That is a great book!! And yeah, they don’t call it ‘Chinese math’ because it is easy!!

Most people don't realize this... percentages work both ways.

10% of 90, is the same as 90% of 10. 9

5% of 50, is the same as 50% of 5. 2.5

If you ever need to figure 22% of 50... it's 50% of 22. 11

That's all I got...

a reply to: TEOTWAWKIAIFF
a reply to: madmac5150

Those are good ones!

a reply to: Tucket

Posting in this thread to leave a trail so I can find my way back and learn more tricks!
I like math tricks, 'cause math can be tricky!!

a reply to: Tucket

Something useless our mother taught us when we were learning our multiplication tables...

How can you tell if a number can be divided by 3 to a whole number?

Add the numbers up.

12 would be 1+2... which equals 3 so is divisible by 3. 12/3 =4.

123 would be 1+2+3... which equals 6 so is divisible by 3. 123/3 =41

12,345,678 would be 1+2+3+4+5+6+7+8... which =36... 3+6 =9, which is divisible by 3. 12,345,678/3 =4,115,226

123,456,789,233,412 would be 1+2+3+4+5+6+7+8+9+2+3+3+4+1+2... which is 60... 6+0 =6 so is divisible by 3.

123,456,789,233,412/3 =41,152,263,077,804

So you can figure out if any number is divisible by 3 by using simple addition in your head.

originally posted by: ByteChanger

originally posted by: GBP/JPY
Please I beg of you someone explain it casting out nines to me.

Down demon, begone now I say! I cast thee back into the eternal pit of darkness from whence you came!

originally posted by: Lumenari
a reply to: Tucket

Something useless our mother taught us when we were learning our multiplication tables...

How can you tell if a number can be divided by 3 to a whole number?

Add the numbers up.

12 would be 1+2... which equals 3 so is divisible by 3. 12/3 =4.

123 would be 1+2+3... which equals 6 so is divisible by 3. 123/3 =41

12,345,678 would be 1+2+3+4+5+6+7+8... which =36... 3+6 =9, which is divisible by 3. 12,345,678/3 =4,115,226

123,456,789,233,412 would be 1+2+3+4+5+6+7+8+9+2+3+3+4+1+2... which is 60... 6+0 =6 so is divisible by 3.

123,456,789,233,412/3 =41,152,263,077,804

So you can figure out if any number is divisible by 3 by using simple addition in your head.

This was the secret of the Trinarians. Then, they ate Pi.

The Bible called it an "apple", but, whatever...

a reply to: madmac5150

This was the secret of the Trinarians. Then, they ate Pi.

The Bible called it an "apple", but, whatever...

twelve times twelve is one hundred forty four. fifteen times fifteen is 225. twenty five times twenty five is six twenty five. I have used those squared numbers many times, but hardly ever use any other squared number over twelve in real life. I could multiply big numbers in my head, but not as good as I used to do before I hit my head on a forklift rack.
Now, I need a calculator for bigger numbers after hitting my head though. I still can multiply two place numbers yet though

a reply to: Tucket

Here’s one that was never explained, just stated, in university math class, of all things, “nobody knows why... but all numbers can be divided by’6’ and show us a property we don’t understand” (why all numbers can be divided by 6).

That was never explained in in our 400+?math course on Discrete Mathematics!!

It is stated as ‘fact’ but not “proven”!!

(It is all about the “remainders” and you end up with [0, 1, 2, 3, 4, 5) and then repeat. Nobody has explained why!).

And that is ‘University math’ at the “just before graduate level” courses!!

There is a lot we don’t know about math!

Besides Riemann, there is Golbach, and Twin Primes,... let alone the Collatz Conjecture (that is so simple to state but nobody can solve it... you may see a thread by me about it... maybe two!)

Math is good for your brain!!



(because we are all drunks!!)

originally posted by: new_here
a reply to: Tucket

Posting in this thread to leave a trail so I can find my way back and learn more tricks!
I like math tricks, 'cause math can be tricky!!

Here's another just for you: if you want to multiply any 2 digit number by 11, there is a trick. Just add the 2 digits of the first number together and insert the sum between the numbers to get your result

For example, take 62*11. 6+2=8, so the result would be 682!

If your sum results in a 2 digit number, add the first digit to the first digit, and insert the 2nd digit in the middle.

For example, take 93 * 11. 9+3=12, so 1+9=10, so the answer is 1023

originally posted by: DictionaryOfExcuses
a reply to: Tucket

I possess no special math prowess. However, sometime during my years I learned a quaint little arithmetic trick which I will present to you now.

Here goes:

1) Multiply any number by 9.

2) Obtain the digital root by adding each digit of the resulting product together. If this yields a sum comprised of two or more digits, repeat the process until you're left with a single digit.

3) It ALWAYS equals 9.

Examples:

A) 5×9=45; 4+5=9
B) 10×9=90; 9+0=9
C) 123,456,789×9=1,111,111,101; 1+1+1+1+1+1+1+1+1=9
D) 987,654,321×9=8,888,888,889; 8+8+8+8+8+8+8+8+8+9=81; 8+1=9

Try for yourself.

To take it a step further..

All you need is 5 pennies and a metal shear. . Bolt cutter etc..


Ok take one penny and chop it in half with your cutter..

Take four pennies and that half a penny and put it in your pocket..

Now we can begin..

(this works well with kids but I got my uncle with it too)

Using your process tell them to pick any number and multiply that number by nine.

Then have them add the digits to come up with 9 again..

Then tell them to half that number..

But not to tell you the number..

It will always be 4.5...

Then bet them You have that exact change in your pocket...

They never expect you to have that half a penny..





Respectfully,
~ meathead

Thanks for the interesting information, now I will be aware.

I'm not good at math. But I always want to learn and educate myself from different sources. Thanks for this thread.