Janie Grace Posted January 6, 2012 Share Posted January 6, 2012 We're at the beginning, Ex. 9. They have taught number bonds with two digit numbers, how to add 2/3 digits in head, how to deal w/ #s close to 100, etc. Suddenly the exercise requires the student to subtract a two digit from a three digit, when the tens in two digit is greater than the tens in the three digit. Examples: 539-70 748-90 353-70 We have not learned borrowing yet. I am thinking that because they are multiples of ten, there is an easy way to do this mentally, but I am having a hard time figuring out how they want it explained. Take the second # away from one of the hundreds? Take it away from the tens and then take the remaining amount away from the hundreds? I'm frustrated. This hasn't been explained. Or maybe it has... I can't find my teacher book. Usually it's self-explanatory, what/how to teach. Help??? Quote Link to comment Share on other sites More sharing options...
Snickerdoodle Posted January 6, 2012 Share Posted January 6, 2012 This is how I remember teaching it the kiddos: 53 tens - 7 tens Quote Link to comment Share on other sites More sharing options...
petepie2 Posted January 6, 2012 Share Posted January 6, 2012 Isn't borrowing or "renaming" covered in 2A? I know it is in the US version. Quote Link to comment Share on other sites More sharing options...
chepyl Posted January 6, 2012 Share Posted January 6, 2012 Isn't borrowing or "renaming" covered in 2A? I know it is in the US version. We spent a lot of time practicing this in 2a. Quote Link to comment Share on other sites More sharing options...
nansk Posted January 6, 2012 Share Posted January 6, 2012 My dd learnt addition/subtraction with regrouping in SM 1B. But I like Snickerdoodle's explanation too. Quote Link to comment Share on other sites More sharing options...
go_go_gadget Posted January 6, 2012 Share Posted January 6, 2012 (edited) My dd learnt addition/subtraction with regrouping in SM 1B. But I like Snickerdoodle's explanation too. I agree with both statements. I don't have 2B right next to me, though, so I can't check to see if we have a similar example. edited to add: I just checked 2B, and we do have the same problem. The explanation Snickerdoodle gave is exactly the one given in the textbook. Edited January 6, 2012 by go_go_gadget Quote Link to comment Share on other sites More sharing options...
Janie Grace Posted January 6, 2012 Author Share Posted January 6, 2012 Thanks, everyone. I wish I could find that textbook and teacher book. But this thread was helpful. What I ended up doing was saying that if you can't take the number away from the tens (like, you can't do 39-70), think about breaking the hundreds into separate hundreds. Then take the 70 away from ONE of those hundreds. So, with 539, take 70 away from 100, leaving you 30 to add back to the 400 left, plus the 39. Does this make sense or is this a really bad way to explain it??? (I was an English major, can you tell?) He was able to do them this way and get them all right, but I don't want to be telling him something that will be problematic down the road... Quote Link to comment Share on other sites More sharing options...
Cindyz Posted January 6, 2012 Share Posted January 6, 2012 Do you have the Home Instructor's Guide? It should guide you through exactly how to teach it. Quote Link to comment Share on other sites More sharing options...
chepyl Posted January 6, 2012 Share Posted January 6, 2012 You should do some samples with manipulatives. We did this first, then I taught him borrowing in the written problems. He got it very quickly. He did need extra practice with borrowing. Quote Link to comment Share on other sites More sharing options...
letsplaymath Posted January 7, 2012 Share Posted January 7, 2012 Thanks, everyone. I wish I could find that textbook and teacher book. But this thread was helpful. What I ended up doing was saying that if you can't take the number away from the tens (like, you can't do 39-70), think about breaking the hundreds into separate hundreds. Then take the 70 away from ONE of those hundreds. So, with 539, take 70 away from 100, leaving you 30 to add back to the 400 left, plus the 39. Does this make sense or is this a really bad way to explain it??? (I was an English major, can you tell?) He was able to do them this way and get them all right, but I don't want to be telling him something that will be problematic down the road... :iagree: This is exactly how I remember it being taught when I was doing Primary Math 2B with my son. It's nice, because it turns the subtraction into addition, which most students prefer. Instead of "539-70" you calculate "439+30" -- easy peasy! BUT with mental maths there is always more than one way to do a calculation. Another easy way is to "take away in parts" -- so, first take away the 30 tens you have, leaving 509 with 40 more to take away. Then, if the student knows his number bonds for 10, it's fairly easy to see that you will end up with a something-60-something (ignoring the other digits momentarily to focus on the tens, taking away 4 tens will leave 6 remaining), so the answer must be 469. The one thing I would NOT do is try to practice "borrowing" in my head. Way too much trouble! I know some people do it that way, but it's too many numbers for me to keep in mind at once. Quote Link to comment Share on other sites More sharing options...
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