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Using mental math strategies to subtract: counting on, counting tens, then ones

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today we are working on lesson 1.9 it is mental math strategies for subtraction and I'm going to do things a little different from what is in the book after work through paper or you can do it on the strategies that I'm doing so well problems together that are not in the book If I don't like them and then I'll show you some problems in the book that you can practice the strategies on so let's start with this problem it says Sam had 46 he went to a car show and got some more for his collection now he has 73 cards how many cards does samba at the car show well before I do anything as I've got to think about my problem and I have to think about what my question is asking me so I want to know how many cards is Sam buy at the car show my problem doesn't tell me my problem get my important information I see he had 46 baseball cards then he went and he got some more so he added some more and now he has three cars so he's got 73 cards total his first part was 46 and I don't know what part he bought at the car show that's what I have to figure out subtract 73 - 46 or I could use some strategies to help me so if I know that 46 + 73 I can count on using a number line so if I think of 46 let's see I know that's close to 50 how many hops does it take me to get from 46 252-4637 4950 so it takes me for Hops and then from 52 6010 Hawks 6270 would be 10 Hops and since 73 is my target I can count by one's 71 72 73 and not cops so I can app I took I know Tim + 10 is 20 4 + 3 is 7 or 27 total hops so 6046 + 20 must be 73 so with subtraction sometimes adding is your best strategy all right so I'm going to erase this part down here and let's talk about another strategy that silly to counting on on a number line but maybe you can do this next Friday a little bit easier in your head since that's our goal of mental math so we can add 10 to and then we can add one so what stinky Rhino 46 if I add that's 56 if I add 10 more that's 66 if I had 10 more that 76 but that that's too far over 73 I need to get as close to 7:30 as I can so 46 + 10 + 56 + 66 that's 25 can count by 1 66 plus what equals 7266 plus 470 70 plus what equals 73 that's 3 so then I can count my two tens 46 + 20 20 + 4 and then 24 + 3 is 27 another problem and let's use these subtraction strategies the Juanita counted all of her and somewhere not she had 82 crayons in all 57 were not broken how many were broken so my question is how many well I think she has 82 total part of those were not broken that part is 57 so I want no the part that was broken so I know have a missing part when I know my whole part and I seen missing part I know I can subtract I can subtract 87 - 50 or skis me 82 - 57 what if I know that 57 plus something equals 82 I can help me out 57 plus something equals 82 because if I think of my equation as a fact family 82 - 57 equals that same I could count on using a number line and I could see how I could get from 57 to 82 so I like counting ones and tens 57 to the nearest 10 57 58 59 60 to get to 60 now I want to go to 80 I said County since I know that's 20 to get from 6280 cuz 6270 is 10 + 10 more so 20 and then one to Hops from 80 to 82 can add my hops together 3 + 2 is 5 and then 20 + 5 is 25 so 82 - 57 we change colors here so it will stick out by 3:25 because 57 + 25 equals 82 now let's see we reply if I added tens and then I added once erased on this right here the strategy would look like move the number line that maybe your favorite mental strategy so I can add 10 and then I could add ones so let's think I'm starting at 57 57 + 10 would be 67 + 10 more would be 77 chadmore another 10 no because then that would be 87 so now you start counting by ones so 77 to the nearest that would be 80 that be 7718 lb 78 79 80 that would be three and then 8282 I'm running out of room I'm so sorry pops so then I can add what I counted on 10 + 10 is 2020 + 3 is 23 23 + 2 is 25 so this is another way that you could add your tens and then add you want to count on so it's a great way to use addition to help you with subtraction alright welcome back so let's take a look at our let's take a look at the practice pages in your book on page 57 you have some problems to work on it you can use what ever my strategy works for you to subtract your problems and to work on your problem solving if you'd like to try the mental math strategies you do that if you want to do the recruit you can do that also use a strategy that works for you try the connect to reading on page to see 8 and I'm not sure if this practice and homework page 15 find and practice and homework page 65 not sure if it has been ripped out of your book or not if it is in your book you can go ahead and practice your strategy on your practice and homework page if it is ripped out and you don't have it it's okay don't worry about it we'll have plenty of time to practice subtraction so I hope this helps let me know if you if you have any questions have a good night

Uploaded by Lori Wilkerson.

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