[PenultimateManStanding]

Just curious about how well folks can do math in their heads. Can you work numbers without paper, without calculator? I know an accountant who doesn't remember anything about fractions. Students these days seem so inept and can't even use a calculator right, let alone use paper and pencil, or god forbid, do it in their head!

[smallpoxgirl]

That's no sweat. For a real challenge, try to estimate the square root of 374 in your head.

[PenultimateManStanding]

Yes, I know, but I want to get a sense of how people feel about just the simple stuff. (for the square root problem, I would approach it like this for mental math: 20x20=400; for 19x19 I figure 20x19=380 then subtract 19 for 361. so it's 19 point something. Past that I get my calculator or at least paper and pencil though I probably could get the nearest 10th before getting uncomfortable and lazy).

[smallpoxgirl]

It's not that hard. You just have to be able to break it down.

First 20x20=(2x10)x(2x10)=(2x2)x(10x10)=4x100=400

(a-b)x(a-b)=a^2 - 2ab + b^2
Therefore
19X19=(20-1)X(20-1)=20^2 - 2x1x20 + 1^2=400 - 40 + 1=361
19.3x19.3=(20-0.7)X(20-0.7)=20^2 - 2x0.7x20 + 0.7^2=400 - 28 + 0.49=372.5
19.4X19.4=(20-0.6)x(20-0.6)=20^2 - 2x0.6x20 + 0.6^2=400 - 24 + 0.36=376.36

Therefore the square root of 374 is about 19.35
(Actual answer from calculator 19.33.)

[PenultimateManStanding]

Nice trick, but it's alot harder to do it mentally than with at least paper and pencil and it would probably be tough for an approximation to the nearest hundreths. Can you imagine what a 18th or 19th century mathematician would think of a nice little HP calculator?

[bobcousins]

$this->bbcode_second_pass_quote('PenultimateManStanding', 'N')ice trick, but it's alot harder to do it mentally than with at least paper and pencil and it would probably be tough for an approximation to the nearest hundreths. Can you imagine what a 18th or 19th century mathematician would think of a nice little HP calculator?

[PenultimateManStanding]

$this->bbcode_second_pass_quote('bobcousins', '
')Depending on the person, I can imagine a range of reactions from heart attack to being treated as the devil.

[smallpoxgirl]

$this->bbcode_second_pass_quote('PenultimateManStanding', 'N')ice trick, but it's alot harder to do it mentally than with at least paper and pencil

[arocoun]

This is a really bad place to look for people who answer that they can't do that problem without a calculator.

Most people here are very capable of independant thought, have a relatively strong desire and ability to learn, and are capable of understanding stuff like EROEI and graphs--these are things things that more than half of people can't do. So, people here are most likely to have higher than average intelligence, especially in mathematics.

Try the same poll in a forum dedicated to the more "average" person (for example, a sports fan forum, or a Democratic/Republican party forum, for example) and you'll probably get vastly different results.

[smallpoxgirl]

$this->bbcode_second_pass_quote('arocoun', 'T')ry the same poll in a forum dedicated to the more "average" person (for example, a sports fan forum, or a Democratic/Republican party forum, for example) and you'll probably get vastly different results.

[UIUCstudent01]

Easy as Pi.

[PenultimateManStanding]

$this->bbcode_second_pass_quote('arocoun', '
')Try the same poll in . . . a Democratic/Republican party forum . . . and you'll probably get vastly different results.

[Jake_old]

I chose sure no problem but I did use my fingerues for the 9x table bit, so not really done in my head.

[Aedo]

$this->bbcode_second_pass_quote('smallpoxgirl', '')$this->bbcode_second_pass_quote('PenultimateManStanding', ' ')Can you imagine what a 18th or 19th century mathematician would think of a nice little HP calculator?

[peaker_2005]

I haven't done this sort of thing in a LOOONNGGG time.

I DO know that the ninths are the ones with the repeating digit - and that digit being the numerator (I think that's the right term. I'm a little rusty).

In other words, 5/9 = 0.5555555555555555r

(where r is shorthand for repeater).

My gut said 20 or thereabouts (it's just a little more than half of 36.

My first thought was unitary method (ie that if 100% is the complete amount, work out 1% then multiply by 55.55555r to get how much it is.. That got 19.8.

Averaging the two gives 19.9.

The calculator bore out 19.999999999999999999999999998 or something similar when I kept putting in 5's after the decimal place as opposed to it being rounded by the calculator.

Rounded, my gut was spot on.

Oh, wait, I wasn't supposed to put this up, was I?



Oh well. Too late.......................NOW1

[PenultimateManStanding]

How about dividing 45 by 5/9ths? You wouldn't have to make them much harder before alot of smart people would scratch their heads. People forget this stuff, because of all the prosthetic brains we have working for us. How about multiplying 84 times 3/7ths? (I know, simple, no problema, but I think some of you would be suprised!)

[Andrew_S]

$this->bbcode_second_pass_quote('PenultimateManStanding', 'H')ow about dividing 45 by 5/9ths? You wouldn't have to make them much harder before alot of smart people would scratch their heads. People forget this stuff, because of all the prosthetic brains we have working for us. How about multiplying 84 times 3/7ths? (I know, simple, no problema, but I think some of you would be suprised!)