When someone asks you to add 48 + 27, what happens in your head?

213 Replies

Thanks so much for responding. I saw this question elsewhere. The standard "math person" way seems to be what @Russell Brazil and @Dawn P. do. @Jill F. does it the way it was taught to me in my NY primary and secondary school experiences. @Mike Dymski  and @Matt Groth  take what I see as the European approach to carrying over (guys, it may just be how basic math was done in your state or local curriculums, IDK).

@Karl B. and @David Lee Hall, III both use the same basic mental math shortcut approach.

But here's the thing, guys -- I cannot help but look at that and see 48 hours as two days and 27 as one day (24 hours) plus three hours. So I do 48 + 24 = 72 (three days), then add 3. It just seems inconceivable to me that people would do it mentally any other way. And I'm not saying my way is better, I'm wondering why it works that way for me.

Originally posted by @Jim K.
I cannot help but look at that and see 48 hours as two days and 27 as one day (24 hours) plus three hours. So I do 48 + 24 = 72 (three days), then add 3... I'm wondering why it works that way for me.

I think you would want to see why you turn the numbers into days. Did you used to ever be on a very regimented schedule (military maybe)? If you can identify that, you may know why you do it with simple addition. I could see if you always moved cases of a product as a supplier for 30 years, that you would do mental math in “cases” as it would be natural to you (a case could be 20, 24, 30, etc.).


Originally posted by @Jim K. :

I'm really curious to see how other DIY investors and landlords answer this mental addition question versus the general population.





(or I excel it).

@David Lee Hall, III

Not military, but it's probably linked to time management. I have renovation periods now where I have to manage my time very well, and structured time management was a regular part of my life for quite a few years as a teacher. But it's weird as hell to see the outcomes like this.

Originally posted by @Jill L Ripley :

@Jim K. I take 2 from 27, to make 48= 50 and whala adding 25 is easy peasy to get to 75🧐

Yes, that's the good-at-math mental shortcut approach. This is why my right hand's fingers are so crooked and I'm just glad I'm not missing any of them.


Using the INSANE NEW math my grandsons public school enforced he would have had to decompose the numbers AND show his work or get a zero regardless of the solution.

        48      +     27    =

      /     \         / \

  40          8   20       7

        40    +    20    =  60

           8    +      7   =  15

          60  +   15  = 85

His public school went 'covid-hybrid-zoom', so now we send him to a private school where they teach him how to solve MATH the way we did in the dark ages. His has since excelled in math, reading & ALL aspects of elementary ed. Then again the class size is 8 not 35.

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