B2.3 Use mental math strategies to multiply whole numbers by 0.1 and 0.01 and estimate sums and differences of decimal numbers up to hundredths, and explain the strategies used.
Activity 1: Distance Travelled by Nassim
Present the following problem to students:
Nassim is a fast walker. He walks 4 500 m per day. Express the distance Nassim walks in hectometres.
Allow students to solve the problem using the mental math strategy of their choice.
Example
I know that 1 m is equal to 0.01 hm. When I want to convert 4500 m to hectometres, I have to multiply 4500 by 0.01, which is the same as dividing by 100.
To perform \(4\;500\; \times \;0.01\), I visualize the number 4500 divided into 100 equal groups.
First, I break 4500 down into \(\left( {100\; \times \;40} \right)\; + \;\left( {100\; \times \;5} \right)\).
So in each of the 100 groups, there are 45.
The distance covered by Nassim is 45 hm.
source: En avant, les maths! 5e année, ML, Nombres, p. 6-8.
Activity 2: What Can I Buy?
Review the list of estimation strategies that students know.
Ensure that students grasp the connection between estimation strategies and everyday situations by discussing "friendly numbers" strategy of using more manageable numbers to make estimations (for example, using 80 instead of 83, or 200 instead of 192).
Project Appendix 3Q.5 (At the Grocery Store) or create similar price lists or create a mini grocery store using plastic food products and empty food boxes.
Explain to students that they have $10 to buy snacks for the whole class and can only total their purchases by estimating.
Encourage them to estimate using "friendly numbers", namely numbers that are easy to handle.
Invite students to explain their approach.
Ask students questions such as:
- What mental math strategies can you use? To do what?
- How were the numbers estimated?
Source: Guide d'enseignement efficace des mathématiques de la 1re à la 3e année, p. 199.