Kevin's Watch

I learnt how the ancient Egyptians used to multiply yesterday. I thought my delight in numbers was long gone, but this brought it all back!

Take the two numbers that you want to multiply and write them down side by side. Double the one on the left and write that number beneath. Halve the one on the right and write that beneath the number just halved. ie

65.......86
130.....43

Keep doing this, doubling the one on the left, and halving the one on the right. If the number on the right is odd, like 43 above, put a * next to it, and when you halve it, forget about any fractional remainder. Keep doing this until the number on the right is 1 (which you'll also put a * beside, because it's odd). You end up with....

65............86
130..........43*
260..........21*
520..........10
1040..........5*
2080..........2
4160..........1*

Now add the numbers in the left hand column, but only the ones where there is a star on that line, so 130 + 260 + 1040 + 4160 = 5590, which is what you get when you multiply 65 by 86.

Ta da!

You have just, apparently, broke these number down into base two (binary), to do multiplication the different way.

That's really great, I'm going to try and remember it. Where did you learn it?

I also love this method, which is apparently taught in China:
www.youtube.com/watch?v=VBsC0p9mHPs

I also love this method, which is apparently taught in China:
www.youtube.com/watch?v=VBsC0p9mHPs

OMG



Edit: mine's was described in a newspaper yesterday

Stonemaybe wrote: 65............86
130..........43*
260..........21*
520..........10
1040..........5*
2080..........2
4160..........1*

...

You have just, apparently, broke these number down into base two (binary), to do multiplication the different way.

Pretty cool... read down the far right side, put a "0" for no star and a "1" for a star, and the binary connection becomes much more apparent

Uh...yeah...I can barely do math the easy way...

--A

There's an easy way to do basic math with your fingers...anyone ever seen that? [heh...I don't mean counting...that would be silly]

There's an easy way to do basic math with your fingers...anyone ever seen that? [heh...I don't mean counting...that would be silly]

I vaguely remember a method to find out which months have 30days by counting knuckles, but can't remember it....

Mathematics can be mind boggling. Fantastic post.

Stonemaybe wrote:I vaguely remember a method to find out which months have 30days by counting knuckles, but can't remember it....

Count knuckles and spaces between.

Vraith wrote:There's an easy way to do basic math with your fingers...anyone ever seen that? [heh...I don't mean counting...that would be silly]

On Northern Exposure, Holling knew how. That's the only time I've ever heard of it. I didn't assume it was a real thing, though.

Isn't that what calculators are for?

Fist and Faith wrote:

Vraith wrote:There's an easy way to do basic math with your fingers...anyone ever seen that? [heh...I don't mean counting...that would be silly]

On Northern Exposure, Holling knew how. That's the only time I've ever heard of it. I didn't assume it was a real thing, though.

Wow...I completely forgot that about him! I did some searching around, there are actually several methods, most of them you have to be able to multiply up to 4 in your head, there are 2 for doing x5-x10, and a different one for any number...

come on then, spill the beans!

Me? Heh, I wasn't trying to be mysterious: here's one of them:

gwydir.demon.co.uk/jo/numbers/finger/multiply.htm

Vraith wrote:Me? Heh, I wasn't trying to be mysterious: here's one of them:

gwydir.demon.co.uk/jo/numbers/finger/multiply.htm

I like!

It is easier, however, just to have had the times tables drummed into you as a kid!

Cool stuff!

Fascinating topic, Stone!

Never knew about that Chinese method either. Thanks, Waddley!

Have to say, they look like the Rube Goldberg way of doing multiplication.
I think I'll stick with the way I learned it in school here, thanks.
(Basic math, the ABC's, and typing -- the only useful learning I got out of school, but that's another topic.)

(this question is out of bounds to anyone who has read 'A Curious Incident....')

You are on a game show on television. On this game show the idea is to win a car as a prize. The game show host shows you three doors. He says that there is a car behind one of the doors and there are goats behind the other two doors. He asks you to pick a door. You pick a door but the door is not opened. Then the game show host opens one of the doors you didn't pick to show a goat (because he knows what is behind each door). Then he says you have one final chance to change your mind before the doors are opened and you get a car or a goat. So he asks you if you want to change your mind and pick the other unopened door instead. What should you do?

What's "A Curious Incident...?" sounds like one of those Lemony Sniksomething books....
Anyway I think, if I recall the math correctly the answer would be:

Spoiler

Better odds of winning car if you switch.

A Curious Incident = Mark Haddon's novel 'The Curious Incident of the Dog in the Night-time'.

Spoiler

Correct answer! If you switch, you are twice as likely to win the car!

I have heard this said on a movie once, but I don't understand the reasoning behind it. What is the logical explanation?