B2.3 Use mental math strategies to multiply whole numbers by 10, 100, and 1000,divide whole numbers by 10, and add and subtract decimal tenths, and explain the strategies used.
Activity 1: Playing On the Island
Material
- Playing On the Island - Rules of the Game (one copy per group)
- Playing On the Island - Game Board A ( \(\times 10\)) (one copy per group)
- Playing On the Island - Game Board B ( \(\times 100\)) (one copy per group)
- Playing On the Island - Game Board B ( \(\times 1\;000\)) (one copy per group)
- number cube with the numbers 3, 4, 6, 7, 8 and 9
- number cube with the numbers 4, 5, 6, 7, 8 and 9
- 40 tokens of two different colours (20 tokens of the same colour per person)
Number of players : 2
Directions
Students choose game board A or one of the game boards B. In turn, each person:
- roll the dice;
- multiplies the resulting numbers (for example, \(4\; \times \;7\; = \;28\));
- multiplies the result by the factor indicated on the chosen game board (for example, \(28\; \times \;10\; = \;280\) on board A or \(28\; \times \;100\) on board B or \(28\; \times \;1\;000\) on the other board B);
- places a token on the square corresponding to the final product on the game board.
Note: If there is already a token on the square, the person skips their turn.
The first person to line up three tokens on the island is the winner. The three tokens can be placed in three ways.
If no one has been able to line up three tokens and all the tokens have been placed, the game is a draw.
Sources:UPBALF Numération et sens du nombre 4e, Module 2, série 2, p. 203-205 UPBALF Numération et sens du nombre5e, Module 2, série 2, p. 197.
Activity 2: Problems to Solve
Have students solve the following problems:
- Patrick says that \(5\; \times \;100\) is equal to \(50\; \times \;10\). Is he right? Explain your answer.
- Nicole says that \(6\; \times \;1\;000\) is equal to \(60\; \times \;100\). Is she right? Explain your answer.
- A number is a multiple of 100, but it is not a multiple of 10. Is this possible? Explain your answer.
- Suki says that \(400\; \div \;10\) is equal to \(40\; \div \;1\). Is she right? Explain your answer.
- Hugo says that 8000 mm represent 8 m. Is he right? Explain your answer.
- Braden writes this equality on the board: \(5\;{\rm{m}}\;{\rm{ = }}\;{\rm{50}}\;{\rm{dm}}\;{\rm{ = }}\;{\rm{500}}\;{\rm{cm}}\;{\rm{ = }}\;{\rm{5}}\;{\rm{000}}\;{\rm{mm}}\). What do you notice?
Source: UPBALF Numération et sens du nombre 4e, Module 2, série 2, p. 206.
Activity 3: Addition of Decimal Numbers
Perform addition using a mental math strategy:
\(\begin{array}{l}1.\;69,7\; + \;24,5\; = \\2.\;61,7\; + \;34,3\; = \\3.\;22,1\; + \;63,4\; = \;\\4.\;17,9\; + \;68,7\; = \\5.\;193,6\; + \;357,5\; = \end{array}\)
Activity 4: Subtraction of Decimal Numbers
This interactive activity allows students to familiarize themselves with strategies for subtracting decimal numbers as well as with various models.
Source: L'@telier - Ressources pédagogiques en ligne (atelier.on.ca).