So today a friend tagged me in a FB post regarding the “frightening” method that students MUST solve subtraction problems. I have posted the link below, and I believe the link is at the bottom of this post as well! Take a look.
So let’s summarize the tutor’s concerns. 1. That we are writing problems horizontally rather than vertically. 2. That students are using a strategy of “adding up” rather than “stack and subtract”. 3. They MUST use this strategy and no other.
I would like to address each of these and provide some comments.
- Most of us are used to seeing math problems vertically. Why? Well, for one, it makes the problem ready to go for the algorithm of “stack and subtract” (which is not the ONLY algorithm in the world, mind you). I would contend it also saves space for publishing companies in their workbooks. If the problem is already written for the algorithm, publishers do not have to provide additional space and therefore can fit more problems on a page, and save money. (Yeah, I went there. Bring on the comments!) However, when you write a subtraction problem vertically, you lose the essence of the numbers. Here I only see single digit subtracted by single digit. I do not see 40 – 20, but 4 – 2 and 5 – 1. This is okay for problems where we do not have regrouping. However, some students get so stuck in the process of regrouping that they no longer see the value of the places and just write a very ‘random’ value as the difference. When I write the problem horizontally (45-21), it allows me to view it from a place value perspective. My eyes look first at the tens and then the ones, versus the horizontal example where I start with the ones and then look at the tens. Also, writing it horizontally does not constrict me to a “stack and subtract” method. (See prior blog for more info on great subtraction strategies to help kiddos.) Really, both are fine; no need to get all uppity about it. And if a teacher says they HAVE to write it that way, it is not true, but what is true is that they learn to think about relationships and different strategies using place value and properties of numbers BEFORE learning the standard algorithm. In fact, the standard algorithm for multi-digit subtraction should not be mastered until grade 3: CCSS.Math.Content.3.NBT.A.2
Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction. So students have lots and lots of time to process what it means to subtract and its relationship to addition.
- (and 3.) Adding up is one strategy students can use. Makes sense to me! If I already know how to add, I can simply use that to help me figure out the missing addend. This is all a subtraction problem really is: a missing addend problem! Consider the following: 43 – 16. This is really finding out 16 + ____ = 43. Now typically, students do not need to go to the next five as the tutor suggests. And really a number line is FABULOUS for modeling adding up. Here is one way to get the value. Notice this is a great strategy for students who struggle with regrouping, because there IS NO REGROUPING!!! I went up 4 to the nearest ten (20), added 20 more (40), and ‘hopped’ 3 more to get to my end point (43). 4 + 20 + 3 is 27. Therefore, 43 -16 = 27. Using the tutor’s problem (43-13), I think adding up is efficient, if you move up by tens. I can simply add by tens until I reach 43. 3-tens is 30. Not sure why it is so convoluted in her explanation and NO! Students do not have to add up the same way the tutor did. In fact, that is the wonderful thing about Common Core. CCSS.Math.Content.2.NBT.B.7
Add and subtract within 1000, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method. Understand that in adding or subtracting three-digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds. Notice that students can use any method, with or without drawings or concrete models!!! Super awesome! Hooray for creativity! I no longer have to use one method (that I don’t really get and just memorized because my teacher told me to); I can use any method so long as I keep in mind place value and my rules (properties).