Adding and Subtracting with decimals

6 teachers like this lesson
Print Lesson


SWBAT fluently add and subtract decimals using the standard algorithm

Big Idea

Computing with decimals is a life skill used when dealing with money.


10 minutes

Before we begin adding decimals, I want the students to think about how decimals and whole numbers are alike and different.  I’m going to accept any reasonable answers.  The reason I chose these 2 questions is because adding and subtracting whole and decimals work the same way except for the placement of the decimal.  I want the students thinking about the connections between the two and how they apply to computations.

Students can do a thinkpairshare to discuss their thoughts. (SMP 3)

Adding and Subtracting Decimals

30 minutes

I will be modeling for students how to add/subtract with decimals.  I will begin by asking them to estimate what their answer is going to be and I will do this for each problem.  Then I will set up the problem and solve.  Each time I do this, I’m going to ask the students where they think the decimal should be placed in the solution and why? (SMP 1 and 6)  Students have a difficult time when it comes to adding and subtracting with a whole number and a decimal.  They forget about where to place the decimal.  I do a couple of things to help out with this.  First, I always have the students estimate their answer which helps with decimal placement in the solution or I remind the students that decimals are like money so if I’m talking about 10, it means $10.  I ask them how $10 is written and they come up with 10.00.

I don’t want the students knowing the steps just yet.  I want them to come up with this on their own.  As we finish adding and subtracting with decimals, I’m going to ask them what they have noticed about “how” to add and subtract with decimals. (SMP 8) I’m going to have students do a think-pair-share before we discuss as a whole group the steps to take when adding or subtracting with decimals. Students can share with their partner what they think is the proper way to add or subtract. I will be looking for students to notice that we need to line up decimals and the reason we line up decimals is because we align place value, just like whole number addition and subtraction.  Once they’ve discussed this, have them share as a whole group.  Students can add the steps to their notes. 

Numbered Heads together

35 minutes

The students will be working on word problems involving adding and subtracting decimals. I chose to use Numbered Heads Together because it’s a good way to have students start working independently and then receiving assistance if needed.  The students will need to decide whether the problem is asking them to add or subtract (SMP 1 and 2) and then use their knowledge of computing with decimals to come up with a solution.  Students will need to prove their solution is correct either by using estimating or re-working the problem in reverse (SMP 3) and use correct units of measurement (SMP 6)

Resources:  whiteboards and markers. 


10 minutes

I want the students to write their own addition or subtraction problem using decimals.  They should write the problem and solve it.  If time permits, they can share their problem and solution with a tablemate.