Subtracting Decimals

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Students will be able to subtract numbers with decimals.

Big Idea

Now that’s a bargain!


10 minutes

Today’s lesson is part two of my extreme couponing series that focuses on subtracting numbers that have decimals.  Students will look up prices of items that they have coupons for and then determine the price after using the coupons. 

I begin today’s lesson by having students write down the items they have coupons for on an index card.  They will need the specific brand and size of each item.

Today we are going to make a list of the items we are going to purchase using our coupons and then go to the computer lab to look up the actual prices of these items.  We you are creating your list please make sure you write down exactly what it is you need to buy. 

Anyone have a coupon for toilet paper?  (I go to a student whom raised their hand.  The coupon is for Quilted Northern 4 pack or larger.)  Alright, perfect.  This coupon is for Quilted Northern four pack or larger.  When I create my list I want to make sure I write both the brand and the size down.  If I pick up a pack of Charmin toilet paper instead and try to use my coupon at the checkout, what do you think would happen?

I allow for student responses and my students are quickly able to make sense of the fact that the coupon for Quilted Northern would not work for the Charmin product.  I relate this back to their list and let them know they can’t just write the item such as toilet paper because there are many different brands.  I give them another quick question of what would happen to focus on the size.  

Similarly, if I bought a two pack of Quilted Northern and went to the checkout, what would happen?  Right, I wouldn’t be able to use the coupon for that either.  So it is important that you write down the brand and the size of the product when you are creating your list. 

After the expectations for this portion are clear to students I bring them to the computer lab and allow them about 15 minutes to look up the prices of their items by doing a google search.  If the students do a google search it pulls up prices and pictures automatically as a search result.  I have students write the prices on the list they created.  Once students have a list with prices on it I have them return to the classroom so that we can begin calculating the new cost of the items they “purchase.”


20 minutes

In this portion of the lesson I will have students chose five items that they would like to purchase using the coupons they have.  The students will be individually calculating the new cost of the item after the coupon. 

Before explaining the activity to students I want to review the rules of subtracting with decimals and do a quick example together.  I ask them to think back to our decimal song video from yesterday.  The video highlighted that the decimal point must be lined up and is brought down into the answer for addition and subtraction.  I ask students to quickly tell their neighbor two important parts of subtracting with decimals.  After a few minutes I bring them back to the whole group and have them share out. 

Alright, bring it back.  Who would like to share with us one of the important parts of subtracting with decimals?  And the other one?  What happens if you have one decimal number with more decimal places than the other? 

Again, students should be able to identify that the decimal point must be lined up and is brought down into the answer.  The second half of the discussion focuses on getting students to realize that sometimes you may have to add place holder zeroes so that you may borrow in the subtraction problem. 

I do an example on the board and have students watch and listen.  Show all my work and thinking for the problem 9.5 – 1.56.  When finished sharing with my students I have them discuss with their partner what three things I needed to do during this problem; line up, bring down, place holder zero. 

I then explain the coupon portion of the lesson to the students. I have students pick five coupons that they are going to use and calculate the new price.  I instruct the students to leave all coupons in the center of the table.

Fifth graders can get a little possessive over things and territorial.  There are not enough coupons for each person to have five.  Additionally, I prefer to not have students arguing over which coupon they want to use.  If the coupons are all in the center of the table each group member has an equal opportunity to “use” that coupon.  Sometimes you just have to plan ahead to keep the peace!

As students are choosing the items they are going to “purchase” I pass out a sheet of blank paper and tell them that they must show all of their work on this sheet of paper.  When they are finished I should see five subtraction problems.  They should be able to explain their thinking for each problem they completed.   I allow the students about ten minutes to work on the five problems. 


20 minutes

So far in this math unit I have covered rounding, adding and subtracting with decimals.  In the previous portion of this lesson I have covered subtracting decimals so to end today’s lesson I will have students apply the other to concepts to our extreme couponing example.

Now that students have calculated the price of the item after using the coupon, I have them individually calculate the total price of all five items as it would it appear at the bottom of a receipt.  I will then have the students compare their total prices to members of their group. 

Everyone have the new price of each item?  Now that you have the new prices let’s add up the total of all your items as the register would show it at the store.  When you are calculating the total price I want you to once again remember the rules of adding decimals and as always show all your work. 

Once you each have the total price for your five items, I would like you to compare your total price to that of your group members.  You need to create a number sentence using greater than, less than, or equal to using all four totals in your group. 

As an added bonus, I challenge your group to write more than one number sentence using the totals in your group.  Be prepared to share your thinking at the end. 

After about ten minutes I call on a couple groups to present their work and share their thinking.