Multiplying Decimals

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Objective

The students will be able to multiply numbers with decimals using the standard algorithm.

Big Idea

Couponing is a real world context students identify with!

Opener

10 minutes

In today’s lesson students will be learning how to multiply numbers that have decimals.  This is lesson three in my extreme couponing series so students will be applying their knowledge of decimals to calculate the total amount of savings over a year if they were to use the coupons each week. 

I begin today’s lesson by showing students a youtube clip from the Extreme Couponing show on televisions TLC.  This clip highlights the top five biggest savings from the show. 

I have students watch and the video and share with their neighbor thoughts they had after watching the clip.  I ask them to think about if couponing could be an important part of shopping.  After a few minutes I ask students to share their observations of the clip.

Practice

30 minutes

In order to introduce the concept of multiplying decimals I show students a Study Jam from the Scholastic website.  The Study Jams website has numerous amazing interactives to show math and science concepts to students.  The best part is that the examples used in the Study Jams are real world type problems.  There is also a section on the Study Jam that creates a little quiz for students to practice. 

I display the Study Jam on the projector and begin going through the interactive as a whole group.  When I am finished going through the first portion of the Study Jam (Home tab) I have students write the problem down and complete it on their own.  I then call upon a few students to come to the document camera and show their work.

Okay so far who can share their thinking on how we multiply numbers with decimals? 

I use the same process when completing the second portion of the Study Jam (Watch Out! Tab).  After having students share their work I ask students to think about the similarities and differences of multiplying decimals and adding/subtracting decimals.  I have students share responses first with their neighbor and then have them share out to the whole group. 

My goal is that students are able to amass that when you multiply decimals you have to count the number of decimal places in the factors and then put that number of decimal places into the product.  This is different from adding/subtracting decimals in which you have to line up the decimal and bring the decimal down into the answer. 

Closer

20 minutes

Now that students are able to understand how to multiply numbers that include decimals I will have them apply it to our extreme couponing example. 

Let’s go back to our coupon example.  If we were to purchase an item each week for a year and use a coupon each week, what would our total savings be over one year?  Any ideas on how we might find out the total savings?

I ask some probing questions to my students until they are able to see that we need to multiply the savings by the number of weeks in a year.  I don’t want to give them too much information on how to set up the problem because I want to leave that up to them as group. 

I ask each group to select three coupons that they will be ‘purchasing’ each week for a year and using one of their coupons.  They need to calculate the total savings for each item and show all their work.

Each group will be responsible for choosing three items to purchase.  You will need to determine the total savings for each item individually.  When you are finished, be prepared to show your work and thinking through this problem.

I allow the students about ten minutes to work through calculating the total savings for three items and then have them present their work to the class.  I allow other students to respond to the presentation by asking clarifying questions at the end.