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Question Detail
Use mental math to find the difference of 5112 draw or describe the strategy you use
 Math
 3
 Common Core
To teach children how to subtract and learn how to borrow your can use the Following song.
I will write out the words for the problem
255
99
I’ve got 5 they want nine, do I have enough? No
So borrow from the left
Scratch it make in smaller slide one to the right (Show student to scratch the 5 make it a 4 and slide a 1 by the 5)
I’ve got 15 they want 9 do I have enough yes, 159 is 6 (Show student where to write the 6)
I've got 4 they want 9 do I have enough? No
So borrow from the left
Scratch it make in smaller slide one to the right (Show student to scratch the 2 make it a 1 and slide a 1 by the 4)
I’ve got 14 they want 9 do I have enough yes, 149 is 5 (Show student where to write the 5)
I’ve got 1 they want 0, do I have enough? Yes
10 is 1!
156!
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2
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How do you do this
Mentally, there could be many ways to solve this problem. It's important that we give students the opportunity to discover many ways to solve problems like these. I would recommend looking into the "Number Talks" concept as it shows how students develop their own strategies for computation. (1) 5112 I could break the subtraction of 12 into 10 and then 2. So 5110=41. Then 412=39. I might show it by drawing base ten blocks or an open number line.
(2) 5112 I could break the subtraction to land on benchmark numbers. I could do 511=50, then 5010=40, then 401=39. Then I could check that 1, 10, 1 is the same as 12. I might show it on an open number line or with equations.
(3) 5112 I could use my understanding of addition and change it to 12 + __ = 51. Then I could think of 12+30=42, then 42+8=50, then 50+1=51. I added up in parts...30+8+1, so the total amount I added onto 12 to get to 51 was 39. I might show it with base ten blocks, an open number line or equations.
(4) 5112 I could use my understanding of addition and change it to 12 + __ = 51. Then I could think of 12+40 which = 52, then subtract 1 so that it's 51. Therefore, instead of it being 40, the answer is 401=39.
And I'm sure there are other ways that kids would do it...they find a way to surprise us!!