Understanding subtraction part 2: multi-digit subtraction

May 8, 2020

I 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!

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Jennifer Schoenwald
Jennifer Schoenwald
5 months ago

Great explanation for parents!

Carol
Carol
5 months ago

Great explanation and demonstration. Thanks.

Ann
Ann
5 months ago

Excellent!

Sabina Puri
Sabina Puri
5 months ago

My students and parents will really like this Thanks

max
max
5 months ago

Why not saying 32 – 15 = (3-2) tens and (2-5) ones = 2 tens and -3 ones = 1 tens and (10 – 3) ones = 17 ?

max
max
5 months ago
Reply to  Dan Finkel

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 ?

Marilyn Milton
Marilyn Milton
3 months ago
Reply to  Dan Finkel

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.