Doing Arithmetic In Your Head

[gaming_mouse]

According to these human calculator guys, this is a skill that can be learned -- not an innate rainman type gift.

Does anyone of books/exercies etc that can improve your ability to do this? I'm pretty good at higher-level math, but have never been good at doing arithmetic in my head. It would be useful sometimes at the table.

[Duke]

[ QUOTE ]
According to these human calculator guys, this is a skill that can be learned -- not an innate rainman type gift.

[/ QUOTE ]

Replace "not" with "not only" and you'll be right. Autistic people do stuff like that in a different way than these "human calculator" guys.

Get a book called Mathemagics. The basic idea is to work left to right, and not right to left (like they teach you in school). That simplifies a lot. They'll cover more tricks as well.

~D

[Evan]

[ QUOTE ]
The basic idea is to work left to right, and not right to left (like they teach you in school)

[/ QUOTE ]
Ummm....Hebrew School?

[Duke]

[ QUOTE ]
Ummm....Hebrew School?

[/ QUOTE ]

You seem to be confusing Reading and Rithmetic. Good try.

~D

[pokerjo22]

For the life of me I've never been able to memorize a table of outs. I do the math each time - I just find it easier. Some people are good at arithmetic, some have better memories.

[greenage]

http://images.amazon.com/images/P/0671877550.01._SCLZZZZZZZ_.jpg

[Alobar]

[ QUOTE ]
For the life of me I've never been able to memorize a table of outs. I do the math each time - I just find it easier. Some people are good at arithmetic, some have better memories.

[/ QUOTE ]

Thats kinda like me, I can never remember how many letters in the alphabet there are, so whenever anyone asks, I just add them up! /images/graemlins/grin.gif

[pshreck]

I see paterns in the stock market and Im really rich. Although I tried teaching it to people but they don't get it.

[AngryCola]

http://ia.imdb.com/media/imdb/01/I/62/75/51m.jpg

[jason_t]

It is a learned skill, not a Rainman gift. Start with this (http://www.amazon.com/exec/obidos/tg/detail/-/0313232008/ref=ase_jimloyshomepage/104-3035639-6533567?v=glance&s=books) at Amazon, and follow the links within.

Do you remember the trick in the movie where Rainman is shown 51 cards from a deck and names the remaining 52nd card? That is a trick that can be learned too if you are able to calculate in your head quickly enough. I can send you the details if you like; it takes about a day to train yourself for it. The toothpick trick is bullshit.

What is the extent of your training in higher mathematics?

[housenuts]

jesus ferguson also does that trick

[gaming_mouse]

Duke,

Thanks for the suggestion. jason_t also recommended a book, and there were a whole bunch of other similar books on amazon as well.

Is the one your recommended especially good?

Thanks,
gm

[mmbt0ne]

That trick is amazingly simple, and everyone here should be able to figure it out.

Here's a hint: 1378, although 78 is just fine too.

[AngryCola]

[ QUOTE ]
That trick is amazingly simple, and everyone here should be able to figure it out.

Here's a hint: 1378, although 78 is just fine too.

[/ QUOTE ]

I figured it out by myself after watching "Jesus" do it.
It's not very hard once you figure out how you are going to go about it.

[offTopic]

[ QUOTE ]
Does anyone of books/exercies etc that can improve your ability to do this?

[/ QUOTE ]

Here's one: Be a dork growing up...as soon as I learned fractions in elementary school, I noticed that the freeway signs generally had three numbers on them, and they increased in size as you went downwards. Whenever my parents drove us someplace, I found myself trying to figure out what I needed to multiply each of the first two numbers by so that I could add them and get the third number as a result. Sad. /images/graemlins/frown.gif

[jason_t]

I don't think the trick is complicated at all, especially if you can calculate quickly in modulo arithmetic. I'd say most people on this forum could learn the trick, but it's another thing to come up with it on your own.

I have a different system than you, I guess. Mine involves 1183.

[ilya]

When I was a little kid, I could multiply 2 4-digit numbers in my head. Nothing impressive, but much better than I can do now. I think it's just practice...that's the only difference between what I was doing then and what I (don't) do now. I certainly didn't have any system or method or anything like that.

[sin808]

What a coincidence this should come up. Just today the postman dropped off my new copy of Speed Mathematics by Bill Handley that covers this very topic.

[Phat Mack]

I don't know if it's still in print, but an book I think excellent is Dead Reckoning by Doerfler.

[BusterStacks]

Luckily I learned this in 3rd grade. We used to have to do a sheet of 100 multiplications in under a minute. Now, I just see the answer when I see the problem.

[partygirluk]

I have always been good at mental arithmetic. I never really worked at it as a kid, and was really exceptional. However, as a I got older, I was required less and less to do mental arithmetic, and relied on a calculator. So I would say that, as with everything in life, it is a mixture of natural talent and hard work.

Take an example: What is 71*68?
Most people would not have a clue where to start, but

i) 7*7 = 49
ii) So 70*70 = 4900
iii) So 70*68 is just 4900 * -(2*70) or 4900 - 140 so 4760.
iv) So 71*68 is 4760 + 68, or 4828.

Really was not that difficult was it?

[ClaytonN]

That's how I do it. I can multiple a two digit number by a two digit number easily.

It's when it gets to three digits it's a bitch.

Case example. Few days ago in class we were discussing if there were 364 days in a year or 365.

Thirty days hath September, April, June, and November

4 X 30 = 120

All the rest have 31 except February*

31 X 7 = 217

217 + 120 = 337

337 + 28 = 365! /images/graemlins/shocked.gif

[Patrick del Poker Grande]

[ QUOTE ]
Few days ago in class we were discussing if there were 364 days in a year or 365.

[/ QUOTE ]
Really? There was discussion?

[partygirluk]

[ QUOTE ]

All the rest have 31 except January


[/ QUOTE ]

Fair enough. Meet me at Heathrow airport on February 30th and I'll give you £100,000 in cash.

[ClaytonN]

February, blah.

I have an amazing talent for freudian slips, even with a keyboard.

[Duke]

It was written by one of the profs at a college I attended for a while. He seemed like a really smart guy, and could do that stuff in his head. He was also a very good backgammon player IIRC.

I don't know if it's any better than the other options, though.

~D

[Homer]

[ QUOTE ]
I have always been good at mental arithmetic. I never really worked at it as a kid, and was really exceptional. However, as a I got older, I was required less and less to do mental arithmetic, and relied on a calculator. So I would say that, as with everything in life, it is a mixture of natural talent and hard work.

Take an example: What is 71*68?
Most people would not have a clue where to start, but

i) 7*7 = 49
ii) So 70*70 = 4900
iii) So 70*68 is just 4900 * -(2*70) or 4900 - 140 so 4760.
iv) So 71*68 is 4760 + 68, or 4828.

Really was not that difficult was it?

[/ QUOTE ]

The way I do it is picturing it as a foil -- (70+1)*(70-2) becomes 70^2 - 70 - 2, so 4900 - 72 = 4828.

Another example, 99*99 becomes (100-1)*(100-1) = 100^2 - 200 + 1, so 10000 - 199 = 9801.

-- Homer

[partygirluk]

Or another thing to do is to note that

(x - y) (x+y) = x^2 - y^2

So in the 71 * 68 case, we know that 70 ^ 2 is 4900, so 69*71 is 4900-1 of 4899, from which we just deduct 71 to get the right answer

or

70^2 is 4900, so 72 * 68 is 4900 - 4 or 4986, and we deduct 68 to get the right answer.