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What happens in your head when you do 27+48?

I do 7 + 8 = 15, 20 + 40 = 60, 60 + 15 = 75.

@Xjs

Tough to explain actually, but:

Bigger number + most significant part of smaller number + remainder.

In this case:

48 + 20 = 68
68 + 7 = 75

@fribbledom @Xjs Agreed, this is how I do it as well.

But Sometimes I would do: 27 + 50 = 77. 77 -2 = 75 (depending on how big the numbers are)
@fribbledom Same here, except that instead of bigger/smaller number, it's just the first/second one for me.

So:
27 + 40 = 67
67 + 8 = 75

@Xjs

@Xjs Bei mir passiert das ganz graphisch mit so Bauklötzchen, die sich aneinanderreihen. Und dann geht's:

27 + 8
35 + 40

@Xjs I do 27 + 50 (I know by heart 50+25), and I take off 2

@Xjs I note that 27 needs 3 to become a multiple of 10, so I take the three from 48 and add it to 27:
27 + 48 -> 27 +3 + 45 -> 30 + 45

30 plus 45 is easy enough to do in one step. If it was more complicated I might repeat with, say, multiples of 25, 50, or 100.

30 + 45 -> 75

#27plus48

@Xjs

i do "48 is almost 50, just needs 2, shave that off 27 leaves you 25 + 50 which is easily 75."

@Xjs
27+48
I actually do 7+8 = 8-(10-7) = (something+1)5, something = 2+4, (2+4+1)5=75

@Xjs I evaluate from left to right, updating with carries as I go. It's probably not the best way, but one advantage is if I get interrupted half way (if it's a longer sum for example) then I have a good estimate. If you evaluate right to left you have nothing. That's just post-rationalisation though!

@Xjs No, I have a weak memory so I do 20 + 40 = 60, 7 + 8 > 10, so we have 70, then 10 - 8 = 2, 7 - 2 = 5, 70 + 5 = 75. It's more operations, but each of them is simpler. My brain is fundamentally a RISC. ;-)

@Xjs

30 + 50 = 80

80 - 3 = 77

77 - 2 = 75

then I check it with the method you describe, as a confirmation

(it's b/c if all I need is back-of-envelope, I skip the check for a few steps so I can just keep thinking, but I go back after if it gets any realer)

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