Math(s) question

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Rorschach
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Math(s) question

Postby Rorschach » 31 May 2021, 18:41

What’s 28 plus 47?

Obviously it’s not a hard sum but I’m interested in how you worked it out.

I took 47, added 30 and then subtracted 2.

How about you?
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mudshark
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Re: Math(s) question

Postby mudshark » 31 May 2021, 18:43

Same. Genius.
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Re: Math(s) question

Postby souphound » 31 May 2021, 21:26

ON this particular occasion, I added 20 + 40 (=60) and 8 + 7 (=15). Then 60 + 15 = 75.

That's basically the way I evaluate my total grocery bill before I get to the till. I only get an approximation in that case, but good enough for me.

I have used the method Rorschach used on occasion, but I guess it depends on my mood.
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Robert
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Re: Math(s) question

Postby Robert » 31 May 2021, 22:01

I am a bit weird I guess but just added up the three.
20 + 40 + 15

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Rorschach
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Re: Math(s) question

Postby Rorschach » 31 May 2021, 22:09

Robert wrote:I am a bit weird I guess but just added up the three.
20 + 40 + 15


There’s nothing weird about that. I often count like that and I was a little surprised I hadn’t on this occasion.
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Rorschach
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Re: Math(s) question

Postby Rorschach » 31 May 2021, 22:11

souphound wrote: .... I guess it depends on my mood.


I think that has a lot to do with it. I’m looking for a pattern but I suspect there isn’t one.
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Robert
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Re: Math(s) question

Postby Robert » 31 May 2021, 22:16

Rorschach wrote:
Robert wrote:I am a bit weird I guess but just added up the three.
20 + 40 + 15


There’s nothing weird about that. I often count like that and I was a little surprised I hadn’t on this occasion.



I use you method sometimes but mostly when the figures are bigger/ too complicated to use the method I described.

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C
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Re: Math(s) question

Postby C » 31 May 2021, 23:38

28 + 47 is an interesting one.

If it had been 29 + 47 then most would have gone for 30 + 47 and then made the adjustment

48 + 47 would have leant itself to a 'near double' strategy - either double 47 plus one or double 48 minus one, whatever took your fancy

Probably the most common method deployed here would be to partition:

20 + 40 plus 8 + 7

The thing about mental calculation is it is the numbers that generally determine the method not the operation.

That is, you may find yourself using an addition method to solve a subtraction calculation

For example, when the numbers are close together

451 - 448. Count on 3

Think about these

1. 2001 - 2
2. 312 + 421
3. 13 x 25
4. 352 ÷ 8





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Re: Math(s) question

Postby trans-chigley express » 01 Jun 2021, 01:02

7 +8=15
20 +40=60
60+15 =75

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Re: Math(s) question

Postby Rorschach » 01 Jun 2021, 06:49

trans-chigley express wrote:7 +8=15
20 +40=60
60+15 =75

Interesting. I don't think I'd ever count the units before the tens.
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Re: Math(s) question

Postby trans-chigley express » 01 Jun 2021, 08:37

Rorschach wrote:
trans-chigley express wrote:7 +8=15
20 +40=60
60+15 =75

Interesting. I don't think I'd ever count the units before the tens.

It's because on paper I add up in columns starting with units (7+8 = 5, carry the 1 to the next column) and in my mind I do the same.

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Re: Math(s) question

Postby Rorschach » 01 Jun 2021, 09:07

trans-chigley express wrote:
Rorschach wrote:
trans-chigley express wrote:7 +8=15
20 +40=60
60+15 =75

Interesting. I don't think I'd ever count the units before the tens.

It's because on paper I add up in columns starting with units (7+8 = 5, carry the 1 to the next column) and in my mind I do the same.



That makes perfect sense. I think I imagine numbers like a pyramid and try to build up from the bottom, with the bigger numbers first.


I know what I mean...
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Re: Math(s) question

Postby Rayge » 01 Jun 2021, 16:53

I just looked at and knew the answer straight away
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Re: Math(s) question

Postby Robert » 01 Jun 2021, 18:43

Rayge wrote:I just looked at and knew the answer straight away


Like every body, I think. But even if you think that, there’s a subconcious method.

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Re: Math(s) question

Postby Rayge » 01 Jun 2021, 19:07

Oh, sure, but it's so swift as to be subliminal: if it were, say, two five figure numbers, then I would be conscious to some degree of the process involved
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Re: Math(s) question

Postby der Freiherr » 01 Jun 2021, 20:27

Rorschach wrote:What’s 28 plus 47?

Obviously it’s not a hard sum but I’m interested in how you worked it out.

I took 47, added 30 and then subtracted 2.

How about you?


I write it out thusly:

28
47

I add the 7 and 8 (my fingers tell me it’s 15), carry the 1, add it to the 2 and 4, that’s 7, so 75. Then I doublecheck on my calculator.

Yep, 75.
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Re: Math(s) question

Postby Belle Lettre » 01 Jun 2021, 20:38

Same - I just write down the sum in my head
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Re: Math(s) question

Postby kath » 01 Jun 2021, 20:56

Belle Lettre wrote:Same - I just write down the sum in my head


same same here.

mind ya, i keep score playin cards everyday. i am the score keeper (which should frighten the rest of the world, but i'm used to it.)

however, my brain will NEVER be able to go back to days of yore when i got thru derivative equations.