# How do you add two numbers?

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• 01-04-06, 03:03 PM
jschen
How do you add two numbers?
Just for amusement, let's try something. I am posting two numbers below. Try to add them either in your head or on paper (no assistance from computer/calculator/abacus/counting beans/etc).

314.159
271.828

Which way did you work? Left to right, or right to left?
• 01-04-06, 03:06 PM
jyossarian
585.987
Least significant to most significant, otherwise you have to keep accounting for carryovers.
• 01-04-06, 03:54 PM
jschen
Yeah, that's how most of us were taught. I started adding in the other direction at about age 12. It's about the same speed once you're used to it, but I like having the most useful part of the result first. I can't multiply from left to right, though, if both numbers have more than one digit. Too many carryovers to keep track of.
• 01-04-06, 04:32 PM
my58vw
• 01-04-06, 04:32 PM
HereNT
Where's the option for "I'm too lazy to add two numbers"?
• 01-04-06, 04:33 PM
timmhaan
the interesting thing is that the size of the numbers change the way i do it in my head. for example, if it were just:

123
321

i'd go from left to right. i'd do 300+100, then 20+20, then 3+1. if it's more than 3 digits i have to work it out by going right to left.
• 01-04-06, 04:37 PM
jschen
Quote:

Originally Posted by my58vw

Interesting... sorry I didn't think of that as a possibility in designing the poll.
• 01-04-06, 04:53 PM
cooker
I said left to right, but it's a bit of a hybrid...I looked at the first 3 digits of each and saw there were no carries, so I added them, and then moved on to the smaller ones and saw that only one column required carrying so I kept moving to the right. Had there been a lot of columns requiring carrying, I might have worked from the right to the left.
• 01-04-06, 04:58 PM
cooker
Quote:

Originally Posted by jschen
I can't multiply from left to right, though, if both numbers have more than one digit. Too many carryovers to keep track of.

For multiplying I definitely start large. If I wanted to do 123 X 123, I would start with 100 X 123, file the answer in "RAM" (Robert's Amazing - or Average - Memory), then 2 X 123 X 10, add that on, then 3 X 123, and add that on. Is it 15,129? (keying in calculator) Yes!
• 01-04-06, 05:02 PM
humancongereel
Quote:

Originally Posted by timmhaan
the interesting thing is that the size of the numbers change the way i do it in my head. for example, if it were just:

123
321

i'd go from left to right. i'd do 300+100, then 20+20, then 3+1. if it's more than 3 digits i have to work it out by going right to left.

yeah, that's how i do it. otherwise i lose track of where i am and get confused. i hate math.
• 01-04-06, 05:07 PM
jschen
Quote:

Originally Posted by cooker
For multiplying I definitely start large. If I wanted to do 123 X 123, I would start with 100 X 123, file the answer in "RAM" (Robert's Amazing - or Average - Memory), then 2 X 123 X 10, add that on, then 3 X 123, and add that on. Is it 15,129? (keying in calculator) Yes!

I'm impressed! Back when I cared more about mental computational speed, I tried to multiply large to small, but never got the hang of it. I too often misalign columns in my mental arithmetic going left to right, resulting in some absurd answers. (At least they were so absurd that they were clearly wrong!)

I always feel compelled to look first, too. Sometimes, there will be some trick for doing the computation quickly, so it's worth a quick glance.

By the way, speaking of multiplying, I find that as I age (to the ripe age of, ahem, 26), I'm starting to lose much of my multiplication table. For example, if multiplying a large number by 6, rather than doing so outright, I often find myself multiplying by 2 then by 3, multiplying by 3 then by 2, or multiplying by 5 (usually by multiplying by 10 then dividing by 2) and adding the original. This started happening in high school sometime.
• 01-04-06, 05:19 PM
shikaka
Quote:

Originally Posted by my58vw

thats what i do... cept i can see the numbers in my head so i kinda working out like its on paper cept its in my head.
• 01-04-06, 05:26 PM
KingTermite
Quote:

Originally Posted by my58vw

Me too....but I do each half right to left.

Like OP....I figured out myself around age 12 or so that it was faster to deal with the big numbers first because they were "even" (0's) on the right side. At the end, you only had to worry about the least significant not being an "even" computation.
• 01-04-06, 05:34 PM
InfamousG
Quote:

Originally Posted by jschen
Just for amusement, let's try something. I am posting two numbers below. Try to add them either in your head or on paper (no assistance from computer/calculator/abacus/counting beans/etc).

314.159
271.828

Which way did you work? Left to right, or right to left?

I usually split it at the decimal in that kind of situation.. not sure why. From there, I go left to right

314 + 271 = 585

.159 + .828 = 97[17] = .987

585.987
• 01-04-06, 05:41 PM
mcoine
why are you adding pi and e (x100) anyway?
• 01-04-06, 07:13 PM
Jerseysbest
In my head, left to right, do it all the time for work. Add the whole number then the decimals, or somethign like that. Do something similar for multiplication too.

On paper, right to left, thats how I'm used to doing it.
• 01-04-06, 07:23 PM
MattP.
Quote:

Originally Posted by my58vw

Thats the same with me. If its in my head, ill go left to right, but on paper its right to left
• 01-04-06, 07:23 PM
jschen
Quote:

Originally Posted by mcoine
why are you adding pi and e (x100) anyway?

It's hard to come up with random numbers. 100pi + 100e was as good as any other choice.
• 01-04-06, 07:39 PM
CastIron
I wear two earrings. A plus sign on one and a minus sign on the other. I just pull the proper ear for the equation. You guess what's used for the equal button.
• 01-04-06, 08:06 PM
Stacey
Calculator
• 01-04-06, 08:19 PM
msviolin57

Oh! What? This isn't my classroom? Nevermind . . .
• 01-04-06, 08:26 PM
jschen
^^^ So if there's a decimal point, you want me to start in the middle? ;)
• 01-04-06, 08:54 PM
msviolin57
Quote:

Originally Posted by jschen
^^^ So if there's a decimal point, you want me to start in the middle? ;)

Dang! You got me! Good point!

I teach 1st and 2nd graders, and kids don't get into decimals for another year or two. Can that be my excuse? :o
• 01-04-06, 09:06 PM
cooker
If I'm doing a mental arithmetic that's a bit too big for my brain, it helps to say it out loud...helps me remember the intermediate answers. Drives my wife crazy.
• 01-04-06, 09:14 PM
jschen
Quote:

Originally Posted by cooker
If I'm doing a mental arithmetic that's a bit too big for my brain, it helps to say it out loud...helps me remember the intermediate answers. Drives my wife crazy.

I think out loud often, too. Not just for arithmetic, but for any sort of mental processing. I also "write" on an imaginary piece of paper in front of me (with an imaginary pencil) when necessary.
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