When I see my students subtract mentally, it makes me feel so accomplished as a teacher. This is such a useful skill and it shows that students truly understand our number system. When students subtract mentally it shows that they are flexible with numbers. They can manipulate the numbers to make easier equations. This allows them to subtract mentally with ease. When my 2nd grade students get this skill down, they can mentally subtract with 3-digit subtraction equations.

 

 

So I thought it would be helpful today to share how I help my 2nd grade students subtract mentally with 3-digit numbers. I do that with the 3-Digit Mental Math Strategy.

 

But first it’s important to know that students need a solid foundation of place value before they can master the mental math subtraction strategy. Learn more about how to set this foundation with your students in this blog post here.

 

Students also do best with this strategy when they are fluent with their addition and subtraction math facts. Learn the best principles to get your students to math fact mastery quickly here in this free workbook: The 7 Steps to Ensure Math Fact Fluency

 

Now let’s get into how to best help students subtract mentally with 3-digit numbers.

 

Subtract Mentally

 

Let’s first go over what the mental math subtraction strategy is.

 

There are 3 types of equations to be aware of when you go to subtract mentally. There are equations where there is no regrouping, equations where you regroup in the ones, and equations where you regroup in the tens. When I say “regroup” I mean that the second number in the equation has more tens or ones than the first number in the equation.

 

Let’s look at some examples of those so that it can bring better understanding.

 

No regrouping: 587-234=

 

Regrouping in the ones: 751-425=

 

Regrouping in the tens: 806-372=

 

If equations have them regroup in both the tens and ones, I suggest to students that they choose a different strategy to help them solve. I feel this is appropriate for 2nd grade students. Later in their schooling, they will learn mental math skills for those kinds of equations. But for 2nd grade, the equations I mentioned above are a great place to start!

 

I always have students check to see if they need to regroup and where. For the no regrouping equations, they can just subtract the ones, then the tens, then the hundreds, to get their answer.

 

For the regrouping the in ones, they are going to subtract mentally by breaking apart the smaller number.

 

Let’s look at an example with the equation 436-217.

 

 

7 is more than 6, so we need to break it apart. To make it an easier equation to subtract, we break apart the 7 into 6 and 1. Then we are going to pretend for a moment that the equation is 436-216. This makes an easy equation where no regrouping is required. We can just subtract our ones, tens, and hundreds to get 220. Then we subtract 1 to get 219.

 

Now let’s look at an example where regrouping is required in the tens.

 

 

When there are not enough tens to subtract, break apart the tens number to make an easier equation. Then subtract the other part. 624-132. 3 is more than 2, so we break apart the 3 into 2 and 1. This makes an easier equation where we don’t have to regroup. 624-122. You subtract the ones, tens, and hundreds to get 502. But that’s not the final answer. We broke apart those 3 tens into 2 and 1. We already subtracted those 2 tens, so now we need to subtract one more ten. 502-10=492.

 

For students to get that final answer, it’s important that they know how to mentally subtract 10 from a number. Learn how I teach that in this blog post here.

 

When I teach my students to subtract mentally, I like to use display pages. These are PDF pages that I display up on our whiteboard using our projector. They explain how to do the strategy and give us lots of examples to work together. This helps set students up for success when they get their own practice with subtraction mental math worksheets and other activities.

 

Subtraction Mental Math Worksheets

The mental math subtraction strategy requires some abstract thinking. So, I first give my students subtraction mental math worksheets that are scaffolded and provide the support students need as they are first learning this strategy.

 

 

These worksheets have boxes up above the number they need to break apart. They break apart the number, subtract using the easier equation with no regrouping, and then subtract the leftovers. I also have worksheets that don’t have the boxes. I give these to students when I feel they are ready.

 

When I first give my students these subtraction mental math worksheets, we do a few problems together as a class, then I have them do a few from the page by themselves. I walk around the desks monitoring students’ work and helping students when needed.

 

I take note of common mistakes and misconceptions. Then I call back everyone’s attention and address those. I may model solving a couple more problems to help students fix those mistakes and find more understanding.

 

Then I give students another subtraction mental math worksheet. They do this one on their own and I wait over by our classroom table. When students finish, they line up by the table. I check their work with them and help them fix their mistakes. I love setting up our math lesson in this way because it gives students immediate feedback and I am not correcting work after school. It’s a win-win!

 

Find these subtraction mental math worksheets here.

 

I still want students to get more practice to subtract mentally so I have them complete subtract mentally by breaking apart the smaller number activities.

 

Subtract Mentally by Breaking Apart the Smaller Number Activities

The first activity I have students do is a scoot activity. This is where I have task cards taped up around the classroom. I have students work with a partner. They take a recording sheet with them, walk around the classroom to the different cards, and solve the equations on them using the mental math subtraction strategy.

 

 

I like students to work with a partner for this activity because they can support each other. Plus, it gets them talking about the math. I will hear them say things like “Oh, we need to break apart the 5 into 3 and 2 because the ones number has a 3!”

 

When students finish the scoot activity, I have them check their own work with an answer key I have left out.

 

Then I have them do a fun mental math subtraction puzzle with that same partner.

This puzzle has students line up the different equations and steps to subtract mentally. Again I love having students work with a partner because they can support each other and talk about the math.

 

 

Students think these math puzzles are so fun! They love finding a spot on the ground, spreading out the pieces, and then lining them up to solve the equations. Find these puzzles here.

 

After that, I want to check students’ understanding so I have them complete an exit ticket. This has them rate themselves on how they think they understand the strategy and then they solve a couple equations. Later, I will look at these and take note of students who could use more practice with the strategy during centers.

 

With enough practice, students will be able to subtract mentally. Continue to review this strategy often for your 2nd grade students to find the most success.

 

Find all of the resources I’ve shared in this blog post here.

 

Save money and find these resources in a bundle with all my 3-digit subtraction strategies here.

Learn more about teaching each 3-digit subtraction strategy in this blog post here.