Understanding subtraction part 2: multi-digit subtraction
May 8, 2020I posted a little while ago about the importance of representations in subtraction. Today, I’m sharing a followup video on how to extend that understanding of fractions to the multi-digit case.
When you work in base 10, the algorithms and even the numbers themselves conceal information. (That’s part of what makes them powerful, paradoxically.) But if we start from understanding, we can see why the algorithms work, and extend them into diverse situations.
This video explain how to understand multi-digit subtraction as multiple simple subtraction problems, and how calling altering our representations of numbers makes the subtraction easy to do. It ends by connecting all of this to the algorithm, so that you can see why the algorithm works, and how it makes sense.
Enjoy!
Great explanation for parents!
Great explanation and demonstration. Thanks.
Excellent!
My students and parents will really like this Thanks
Why not saying 32 – 15 = (3-2) tens and (2-5) ones = 2 tens and -3 ones = 1 tens and (10 – 3) ones = 17 ?
You mean 32 – 15 = (3-1) tens and (2-5) ones = 2 tens and -3 ones = 1 tens and (10 – 3) ones = 17?
That’s a great way to do it too. Once you really understand the base 10 structure, there are lots of options.
In fact my question is actually around the notion of negative number and hence see substraction as addition of negative (also called opposite) number https://gdaymath.com/lessons/powerarea/1-4-an-aside-on-negative-numbers-piles-and-holes/.
Is it too confusing to introduce this notion first ?
What about if kids want to do 32 – 45 ? is it forbidden ?
I don’t think it is forbidden, but most students don’t learn this first, and it’s a bit more abstract. Part of my goal right now is to choose one clear approach, so people don’t feel overwhelmed.
I usually have a second grader who comes up with the negative number strategy.
How about using the open number line or the break apart strategy of bridging to a ten?
32-2=30
30-10=20
20-3=17
Because children know their tens combinations, these are easier subtraction problems.