Does it seem like your students are confident with addition skills, but things start to fall apart with subtraction? Especially when the subtraction problem involves regrouping across multiple zeros. Try this easy subtraction trick to steer students to accuracy when subtracting. I have an easy hack for students to subtract when the problem looks like this:
This easy subtraction trick will have your students re-engaging with subtraction in a new way.
Before we get started with the nitty-gritty teacher talk, if you are working on transforming your math classroom into a space where students actively engage in their learning to practice fluency and math concepts, grab my 10 Free Whole Group Math Activities.
No gimmicks, no sales, no nonsense with this freebie- just one way I can help with making math hour the favorite part of the day for every student!
Why Use A Subtraction Trick?
By now, you’ve probably aware of all the work your students have done in second grade with understanding place value before they endeavor to use an algorithm to subtract. Or maybe you are the designated teacher to introduce your students to the algorithm.
Third grade teachers also have the gigantic task of leading their students into the new territory of multiplication and find they don’t have a lot of time to “shore up” subtraction skills. There is pressure to review addition and subtraction quickly during the first few weeks of school and then move on to “3rd grade” math content. If students lack strong number sense, it can feel like stepping into quicksand!
I am not typically a proponent of teaching students math hacks without an ulterior motive. For me, math tricks and hacks have their place in math instruction, but for specific purposes and they need to benefit the overall understanding of numbers, while helping students make connections. Many times, math hacks will illustrate Mathematical Practice Standard #7 Look For and Make Use of Structure as well as Practice Standard #8 Look For and Express Regularity in Repeated Listening. Mathematical Practices – Common Core Math
What I Will Accomplish With This Subtraction Hack
For this subtraction trick, I have two goals. First, I am going to give students a way to improve their accuracy when solving a subtraction problem involving regrouping across several zeros. This hack will involve one step which will turn the problem into a standard subtraction problem where no regrouping is necessary.
Second, I am going to reinforce the important concept that subtraction is the difference between two numbers. If you think your students already understand this, try asking them to define subtraction. They will often describe subtraction as a process, but not be able to define the concept.
How Do I Introduce This New Hack?
As with anything new, I always spend a minute or two providing a familiar context to students so they have an “entry point” to new learning. I would give an example of this type of problem and we would talk about how they would typically solve it. I would ask them what is difficult about it, what they like or don’t like about it.
Then I would ask them what if we could find a way where we didn’t have to worry about trying to subtract from a zero? Usually students are very interested when it comes to an idea that means less work for them.
Okay, here is the important part, so I make sure I have everyone’s attention. I ask them, remember how subtraction is really only the difference between two numbers? If we take one away from each number, are they the same distance apart?
Sometimes I will model this on a number line. Once I start to see some recognition, I ask them if we can apply that idea to the subtraction problem on the board. If we take one number away from the top and one from the bottom, this should give us the same answer, right?
I take them through rewriting the both numbers after subtraction one from each. Note: Sometimes, students with shaky place value understanding will find it difficult to subtract one from a number ending in 0. We rework the problem, verify that the answer is the same and, suddenly, we have a new math hack. Additionally, it is one more opportunity to cement the concept of subtraction.
Final Thoughts on This Easy Subtraction Trick
Introducing new strategies and methods is illustrating how numbers work together. For a healthy math classroom culture and environment, you want each student to feel comfortable and confident in finding the strategies, and ways of thinking that are unique to them. Even though they may not end up using a new computational strategy, it may be just the ticket to making a rich connection and having one of those “Aha” moments that marks a personal journey in math.
My favorite moments during math class are when students feel empowered to experiment, try different methods, learn what works, and sometimes more importantly, why things don’t work. When students have the freedom to work through “what ifs” and “wonderings” they are on the road to becoming great mathematicians.
For more subtraction ideas, check out Brett Berry’s blog post Techniques for Subtracting With Ease
Jessica Kaminsky has more subtraction tips and hacks at 16 Math Magic Tricks for Kids to Calculate Numbers in a Split Second
Looking for a different way to teach subtraction? Read my latest blog post A Different Way To Teach Subtraction
