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? 
585.987
Least significant to most significant, otherwise you have to keep accounting for carryovers. 
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.

In your head is a little different for me. I add the large stuff then the small stuff then add them together...

Where's the option for "I'm too lazy to add two numbers"?

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. 
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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.

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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. 
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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. 
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314 + 271 = 585 .159 + .828 = 97[17] = .987 585.987 
why are you adding pi and e (x100) anyway?

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. 
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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.

Calculator

You must always start with the ones column!!! Always, always, always!!!
Oh! What? This isn't my classroom? Nevermind . . . 
^^^ So if there's a decimal point, you want me to start in the middle? ;)

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I teach 1st and 2nd graders, and kids don't get into decimals for another year or two. Can that be my excuse? :o 
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.

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