sbgrace Posted September 17, 2011 Share Posted September 17, 2011 When should I expect mental subtraction with regrouping and how should I get there? When does RightStart C expect a child to do subtraction like 43-7 or 43-17 mentally/without the abacus? I've got Activities for the AL Abacus but I can't figure out when the switch is made and how. My Math in Focus (Singapore type) and Math Mammoth both show it on paper (regrouping by crossing out one the 10's and adding a 1 to the ones--the old what I called borrowing) and with blocks first. Is the showing the regroup on paper an important step or should I just do abacus and then move him to mental when he's ready? Is paper with all the crossing out and carrying stuff a good or bad thing? His twin automatically knew how to do it all mentally because he's math inclined but I think as easy as it was with that child it's going to be difficult with this one. Quote Link to comment Share on other sites More sharing options...
nansk Posted September 17, 2011 Share Posted September 17, 2011 The on-paper method is needed more with bigger numbers. You teach it with two-digit numbers so that they master the method first before applying it to four or more digit numbers. My first grader has done it in school using Singapore maths. They used icecream sticks and unifix cubes. She doesn't need the manipulatives now. Last year I have used the Montessori golden bead material to show her grouping and regrouping of upto 4-digit numbers. I think you can wait to teach the on-paper method next year if your ds needs more time. For now, if he gets it with manipulatives, it is enough, imho. Quote Link to comment Share on other sites More sharing options...
Spy Car Posted September 17, 2011 Share Posted September 17, 2011 When should I expect mental subtraction with regrouping and how should I get there? When does RightStart C expect a child to do subtraction like 43-7 or 43-17 mentally/without the abacus? I've got Activities for the AL Abacus but I can't figure out when the switch is made and how. My Math in Focus (Singapore type) and Math Mammoth both show it on paper (regrouping by crossing out one the 10's and adding a 1 to the ones--the old what I called borrowing) and with blocks first. Is the showing the regroup on paper an important step or should I just do abacus and then move him to mental when he's ready? Is paper with all the crossing out and carrying stuff a good or bad thing? His twin automatically knew how to do it all mentally because he's math inclined but I think as easy as it was with that child it's going to be difficult with this one. In Primary Mathematics (Singapore) rather than the type of re-grouping that Americans tend to call "borrowing" they would teach mental math strategies for 43-7. You might split 43 into 33+10-7, which gives you 33+3, or 36. They also introduce the standard algorithm of column subtraction, so students get both, but mental math strategies come first. Bill Quote Link to comment Share on other sites More sharing options...
sbgrace Posted September 17, 2011 Author Share Posted September 17, 2011 In Primary Mathematics (Singapore) rather than the type of re-grouping that Americans tend to call "borrowing" they would teach mental math strategies for 43-7. You might split 43 into 33+10-7, which gives you 33+3, or 36. They also introduce the standard algorithm of column subtraction, so students get both, but mental math strategies come first. Bill Ok. The Math in Focus shows 10/1's charts with a ten converted to the ones column first. Then they quickly show the pencil/paper. So I don't see the mental there that Singapore uses. How would Singapore introduce the 43-17 problem initially? I introduced this topic today intending to use the AL Abacus method of introduction but one son told me he didn't want to use the abacus. He did all of them (down to the 75-18 types) easily mentally. However, he used his own method I guess and it's different than you described. It was quick so I think it's working for him and ok? but when I asked him to describe he was doing (43-17) 33 (took the 10) 33 - 3 and then 4 more so 26. Do you think that's a problem? I wonder if I should have him use paper/pencil? Quote Link to comment Share on other sites More sharing options...
Spy Car Posted September 17, 2011 Share Posted September 17, 2011 Ok. The Math in Focus shows 10/1's charts with a ten converted to the ones column first. Then they quickly show the pencil/paper. So I don't see the mental there that Singapore uses. How would Singapore introduce the 43-17 problem initially? 43-17=23+20-17 23+3=26 I introduced this topic today intending to use the AL Abacus method of introduction but one son told me he didn't want to use the abacus. He did all of them (down to the 75-18 types) easily mentally. However, he used his own method I guess and it's different than you described. It was quick so I think it's working for him and ok? but when I asked him to describe he was doing (43-17) 33 (took the 10) 33 - 3 and then 4 more so 26. Do you think that's a problem? I wonder if I should have him use paper/pencil? His method is totally valid. Discussion of the strategy of the sort you had is vital IMO. I would not squelch "his way," but might discuss alternative strategies. My child at times came up with some very interesting ways of re-grouping. There is no "one right way." I really prefer working (and discussing) these types of problems mentally first before writing them put as standard algorithms because it seems more brain streching, but high facility with mental math can cause some resistance to be overcome when demanding work be shown in standard column addition and subtraction. A good bit of advice I got here (to overcome standard algorith resistance to numbers he could add and subtract in his head) was to throw in more digits until the mental math was too hard. That was good advice. Bill Quote Link to comment Share on other sites More sharing options...
crazyforlatin Posted September 17, 2011 Share Posted September 17, 2011 It was quick so I think it's working for him and ok? but when I asked him to describe he was doing (43-17) 33 (took the 10) 33 - 3 and then 4 more so 26. Do you think that's a problem? I wonder if I should have him use paper/pencil? Perfect! DD uses this way even with bigger numbers, but she has come up with other ways too. We have a name for each method, and this one is called the Secret Way. I would avoid paper and pencil because I didn't see any stretching of the brain with standard algorithm; it almost seemed as if she were thinking of other things while crossing out numbers. It was robotic, mechanical, busywork.... I sound anti-standard algorithm, but I'm not; I just think it's good to have children play with numbers in their heads first. Quote Link to comment Share on other sites More sharing options...
anabelneri Posted September 17, 2011 Share Posted September 17, 2011 Other resources for doing mental subtraction are MEP (at least through Year 2... we're only starting Year 3 now but maybe someone else knows) and the Speed Maths series sold by Singapore. I really liked the way that Speed Maths worked through a variety of ways to take numbers apart in order to do math mentally. :) Quote Link to comment Share on other sites More sharing options...
Soror Posted September 17, 2011 Share Posted September 17, 2011 We are in RSC(just started this year) and I would expect that ds could do that mentally now but it hasn't went over officially- I think maybe partway through this year. Quote Link to comment Share on other sites More sharing options...
Crimson Wife Posted September 17, 2011 Share Posted September 17, 2011 In lesson 86 of RS C there is a lesson on mentally subtracting two digit numbers. RS has the student subtract the tens first and then subtract the ones. The example given is 140 - 83 as 14-ten minus 8-ten equals 6-ten and then 60 - 3 = 57. Quote Link to comment Share on other sites More sharing options...
sbgrace Posted September 17, 2011 Author Share Posted September 17, 2011 In lesson 86 of RS C there is a lesson on mentally subtracting two digit numbers. RS has the student subtract the tens first and then subtract the ones. The example given is 140 - 83 as 14-ten minus 8-ten equals 6-ten and then 60 - 3 = 57. Do you think I should let struggling child just use the abacus now then? When he uses that abacus I don't think he's thinking much. I could introduce the smaller subtractions 14-8 type mentally and then use abacus for the rest perhaps. Quote Link to comment Share on other sites More sharing options...
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