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Adding and subtracting decimals

Adding decimal numbers is fairly straight forward, but still some students manage to produce an incorrect answer.
7.1
PAULA KELLY: OK, so we’re looking now at adding some decimals together. We’ve got our decimal sum, so 4.362 and 12.41. Before we add them together, we should really have an estimation of what size of answer we’re expecting. So what would you think?
25.4
MICHAEL ANDERSON: Well, the first one we could round down to 4, and then we could around the second one to 12. So 4 and 12 would give us 16. I suppose we could round up as well, so we could say, well, if we rounded that up to the nearest whole number, it would be 5. And then 12.41 would go to 13, which would give us 18. So I’m expecting an answer somewhere between 16 and 18.
49.1
PAULA KELLY: OK, very good. It’s really important to have an idea of what kind of size of answer we’re looking for. So if we have a look properly, then, if we’re very careful how we land upon numbers– so we have in our ones column, we have 4, 3, 6, 2. Be really careful how you lay these out.
70.3
MICHAEL ANDERSON: So the units are under the ones column and then keep on going on with the tens, hundreds, and thousands.
76
PAULA KELLY: Fantastic, so we’re going to add onto this our 12.41. So again, our tens, our twelves– line up our decimal– and our 4 and our 1. OK, so like any normal addition, if we start from the far right-hand side– sometimes I encourage students to put a place holder of a zero in there. So we’ve got our 2 plus 0, our 6 plus 1, 3 plus 4– very important to keep our point in there– our 4 plus 2, and our 1.
108
MICHAEL ANDERSON: Excellent.
108.9
PAULA KELLY: So our final answer is 16.772. That’s between our 16 and 18, so we’re pretty confident we got it right.
115.8
MICHAEL ANDERSON: Brill.
Adding decimal numbers is fairly straight forward, but still some students manage to produce an incorrect answer by adding the wrong digits together.
In this video we consider two simple strategies: the importance of lining up the digits in the correct place value to help students get the correct answer, and making an estimate of the size of the answer to help students judge whether their answer makes sense.

Problem worksheet

Now complete questions 5 and 6 from this week’s worksheet.
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Maths Subject Knowledge: Fractions, Decimals, and Percentages

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