Question Detail

Use mental math to find the difference of 51-12 draw or describe the strategy you use

Sep 13, 2016 2:43pm

  • Math
  • 3
  • Common Core

2

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    • Sep 29, 2016 7:14pm

      How do you do this

      • Oct 29, 2016 3:11pm

        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) 51-12 I could break the subtraction of 12 into -10 and then -2. So 51-10=41. Then 41-2=39. I might show it by drawing base ten blocks or an open number line.
        (2) 51-12 I could break the subtraction to land on benchmark numbers. I could do 51-1=50, then 50-10=40, then 40-1=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) 51-12 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) 51-12 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 40-1=39.

        And I'm sure there are other ways that kids would do it...they find a way to surprise us!!