I hate checking the kids' subtraction when they have problems like
5006
-347.
I can do it. I am a mathematician, after all. But right now, my brain is stressed enough keeping up with the laundry and bread-baking, so checking three people's math daily without answer keys is just more effort than I wish to expend.
But you know what I discovered today? (Oh, this is SO cool!)
I'll use the above problem for an example.
Rather than having the 6 run next-door to the 0 to borrow (or "regroup" as we're supposed to say in these modern days), and then have the 0 run next-door to the 0 to borrow, and then have the 0 run next-door to borrow from the 5, and then to trickle the regrouping all the way back,...
have the 6 run-next door to the 500. Borrow 1 from the 500, and it becomes 499. And then you're all set to subtract the rest of the problem!
How easy is that?
Subscribe to:
Post Comments (Atom)
Eh? Oh, I see. Cool! But then I'd still have to subtract 499-347 in my head, which I'd still tackle with the convoluted method below ----> 499 is almost 500; 347 is almost 350. 500-350=150. Now subtract 1 (500-499), then add 3 (350-347) with a result of 152.
ReplyDeleteHere's my attempt the original problem: 5000-350=4650, then add the 6 (from 5006) and then add back the 3 to the 347. So now you have 4350+9 = 4659
It's painfully slow, but it's the only way I can do mental math. No, I wasn't taught to do it that way in school; I was taught using traditional algorithms. Which I never understood.
I do the mental math the same way you described. It may be slow, but it's faster than chasing down the calculator and typing in the numbers.
ReplyDeleteBut since I'm not doing mental math, but grading arithmetic papers, it's written on the page in front of me. Then, seeing all those 9's lined up to subtract from, that's easier than the mental math which I use more often in Real Life. Also faster than punching numbers into the calculator. And when you teach it to a kid this way, less likely for them to ooops on one of the many regroupings they have to do when following the traditional algorithms.
Hm. In a problem as small as that I see that I merely need to take 340 from 5000, which is 4660, and then subtract 1, making 4659.
ReplyDeleteI personally the gimmicky questions easier, because there's usually only one difficult bit to it. If you have a more random question, say, 5884 - 927, there tends to be more to keep track of. (this question took me about 15-20 seconds, while the other took perhaps half that. I'm waaay behind in math though.)