Candy maths: All About Decimals
Students will be able to add and subtract decimals to the hundredths place using a model.
- Tell students that today they will be practising adding and subtracting decimals using a special tool.
- Hold up some bags of M&Ms to get students excited about the lesson!
- Hand out the place value mats to students. Explain that students will be adding and subtracting decimals using this place value mat and the M&Ms.
Explicit Instruction/Teacher modeling(15 minutes)
- Project the place value mat on the board or recreate one that is easy for students to see.
- Show students the number 1.45 by put a yellow M&M in the ones places, 4 red M&Ms in the tenths place, and 5 blue M&Ms in the hundredths place. Write "1.45" on the board.
- Tell students that you now want to add 2.08 to this number. Write "+2.08" next to 1.45.
- Put two yellow M&Ms in the ones place and 8 blue M&Ms in the hundredths place. Ask students why they think you skipped the tenths place? Tell them that there is no value in the tenths place because there is a 0.
- Ask students what is wrong with the place value chart. Students should point out that there are 13 blue M&Ms in the hundredths place, but there are not supposed to be more than 9.
- Ask students what you should do to get rid of some of the blue M&Ms. Refer students to the key at the bottom of the chart.
- Take 10 of the blue M&Ms away. Remind students that these M&Ms can't just disappear, but they are going to be replaced with one red M&M, since 10 blue M&Ms is equal to one red M&M.
- The place value chart should now have 3 in the ones place, 5 in the tenths place, and 3 in the hundredths place. Show students how to write this number: "3.53".
- Clear the board. This time, put 7 yellow M&Ms in the ones place and 1 red M&M in the tenths place. Write "7.1" on the board.
- Tell students that you want to subtract 3.06. Next to 7.1, write "-3.06."
- Go to the ones place. Show students that you are taking away 3 yellow M&Ms.
- Go to the tenths place. Show students that you do not need to take anything away because there is a 0 in the ones place.
- Finally, go to the hundredths place. Ask students how you can take away 6 when there is nothing in that place value. Show students how to take 1 red M&M from the tenths place and replace it with 10 blue M&Ms in the hundredths place. Then, take away 6 blue M&Ms.
- There should be 4 yellow M&Ms in the ones place, 0 red M&Ms in the tenths place, and 4 blue M&Ms in the hundredths place. Show students to write this as "4.04."
Guided practise(20 minutes)
- Hand out the bags of M&Ms.
- Give students the problem "2.9+1.12." Have students model this on their place value mats. Walk around and monitor students. Review the method and answer as a class.
- Next, give students the problem "5.87-2.93." Again, students should model this on their place value mats as you walk around and monitor. Review the method and answer as a class.
- If necessary, give a few more problems for students to try one at a time, with a review directly after. These problems should require students to regroup their M&Ms.
Independent working time(10 minutes)
- Hand out a note card to each student.
- Give students at least 2 more problems.
- Have students work these problems out on their place value mats. They should then record their answers on the note card.
- Enrichment:Advanced students could try completing some multiplication problems on their place value charts, such as 1.7x3 or 3.2x2. Remind these students that multiplication is repeated addition if they need help getting started!
- Support:These students could benefit from some small group practise with the teacher or a peer mentor.
- Use students' note cards to determine if they were able to successfully add and subtract decimals.
- While students are working independently, you could also take anecdotal notes about what you observe. Are students able to regroup? Are they putting M&Ms in the correct place value?
Review and closing(5 minutes)
- Students may eat their M&Ms as they discuss the following questions with a partner: Do you think it is easier to add or subtract decimals? Why? What is the most difficult step in adding or subtracting decimals?
- Have students share some of their answers with the class.