General Discussion
In reply to the discussion: How would you answer this test question? From a 1st grade Common Core test. [View all]thesquanderer
(12,904 posts)932-687
I'd round to the nearest hundreds... i.e I know right away we're looking at "approximately" the difference between 900 and 700. The difference between 700 and 900 is 200.
Add to that how far 687 is from 700. That's 13. (Or if you must, it's 10 to get from 700 to 690, and another 3 to get to 687.)
Add to that how far 932 is from 900. That's obviously 32.
32 and 13 is 45. Add that to the 200, I've got 245, quickly and easily, with no paper.
It's not any kind of "math trick"... it's a logical use of rounding and using those "friendly" numbers (i.e. ones that are easy to manipulate in your head, either because they end in zero or because they are small). And I'll have the answer faster than someone can pull out paper and pencil.
This was not anything I was taught in school... but I would see value in teaching it. Along with the paper and pencil approach.