Tag Archives: Subtraction

Grammithmatic Part 3


BLESSIs there really no such number as Eleventeen? Well, there isn’t, but bear with me for a minute.

I have been posting about how we math educators can play with language and make our slightly awkward number naming system a little more logical and a lot more transparent. Check out Part 1 to read about playing with number names to clarify our base-ten system. Check out Part 2 to read about how we can make place value and zero clearer. In Part 3, I am exploring how to make borrowing clearer to the new learner.

The number eleventeen was derived by some astute and logical child who figured that since 13, 14, 15, 16, et al. all end in TEEN, then so should 11, and 12 for that matter. Of course, the poor dear didn’t count on the linguistic mashup that is English. There is also every chance that some other child called 11 ‘Oneteen’ and 12 ‘Twoteen,’ bless her heart.

Wouldn’t it be nice if our number names made more sense? Well, what’s stopping us?

Read this list of numbers to yourself. Then tell yourself what comes next.

10, 20, 30, 40, 50, 60, 70, 80, 90

Did you follow Ninety with One Hundred? Of course you did.

Now, how about this string? Read it to yourself and say what comes next.

100, 200, 300, 400, 500, 600, 700, 800, 900

Did you say One Thousand? Or did you say Ten Hundred? Which one is right? What’s the difference?

Most would say that One Thousand is technically the correct response, but in common vernacular, Ten Hundred is acceptable too. We usually said Nineteen Hundred Ninety-Something a couple of decades ago, and we all knew what we meant.

My idea is to take this already common and, quite frankly, logical exception to naming a number, and apply it to clarifying the act of borrowing.

Look at this equation.

GRAM3A

If we subtract these numbers in columns, without any deriving, manipulating, or expanding of the numbers (let’s go ‘old school’ for a moment), we immediately end up in a situation where we have to borrow. And how do we do that?

GRAM3B

We can’t do 4-7, so we have to borrow 1 TEN from the Tens column, effectively breaking that decade unit into 10 ONEs. We add those to the 4 ONEs already in the Ones column. Now we have 14-7. But look at how we’ve notated that borrowing on paper. A new learner would look at that top number and lose it. That number now makes no sense. It was 234, now it’s 2214, which looks to me like Two Thousand, Two Hundred Fourteen.

But of course it isn’t actually. It’s 2 HUNDREDS + 2 TENS + 14 ONES.

So here’s my thinking. Let’s actually call it that. The original number was Two Hundred Thirty-four. Now it’s Two Hundred Twenty-fourteen.

TWENTYTWELVE.jpgYou know, as in Twenty-eight, Twenty-nine, Twenty-ten, Twenty-eleven, etc.

If we can say Ten Hundred, why can’t we say Twenty-fourteen?

Typically we abandon what the Minuend (the number we are subtracting from) is called (thereby losing the value) the moment we break it up to borrow. We hash it up, cross things out, write things in. It’s no longer an identifiable number. However, while we broke the rule about how many digits are allowed in a column, and put more than 9 ONEs into the Ones column, we can try to keep order and at least follow the rest of the rules by naming the new number logically.

Rather than jumping in to solve 14-7, I’ve read that it’s a good idea to do all the borrowing before any subtraction is actually done. So, let’s keep borrowing and move on to the Tens.

We can’t take 5 TENs from 2 TENs. We must borrow 1 HUNDRED, break it up into 10 TENs, add them to the 2 TENs already in the column, and get 12 TENs altogether.

GRAM3C

Now, we’ve done all the borrowing we have to do, and the minuend is completely mangled. Any thoughts on what we could call this new number? What do we name 12 TENs sitting in the Tens column?

TWELVETY

Seventy, Eighty, Ninety, Tenty, Eleventy, Twelvety!

Therefore, I propose the new number we are subtracting from is One Hundred Twelvety-Fourteen.

One Hundred minus One Hundred is Zero Hundreds (or No Hundred, if you read Part 2). Twelvety minus Fifty is Seventy. Fourteen minus Seven is Seven. So the ‘answer’ is Seventy Seven.GRAM3D

Without naming the new number and making the borrowing obvious, there is so much potential for confusion. Children don’t understand how the carried over numbers apply. They think that the number has suddenly increased in magnitude. They forget to cross things out…

 

But, do you see how, by naming borrowed numbers this way, we are following through with the properties of our number naming system, staying true to the Hundreds, Tens, and Ones structure of base-ten, using the name system to impose order and make what we did with the borrowing process obvious?

 

Would I exclusively name numbers this way. Absolutely not. That does no one any good. However, by renaming the minuend, we are playing with language. Kids love playing with language. By playing with the language, we are making our number system clearer, exposing rules that have been hidden due to linguistic laziness/pragmatism over time. A clearer sense of number in this gaming context makes the learning stick in a meaningful, more flexible and fluid way. And, when we learn any language, our goal is fluency. The language of math is no exception.

 

 

 

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