At a Glance...

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: Problems to Solve

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Have students solve the following problems:

1. Patrick says that $5\times 100$ is equal to $50\times 10$. Is he right? Explain your answer.
2. Nicole says that $6\times 1000$ is equal to $60\times 100$. Is she right? Explain your answer.
3. A number is a multiple of 100, but it is not a multiple of 10. Is this possible? Explain your answer.
4. Suki says that $400÷10$ is equal to $40÷1$. Is she right? Explain your answer.
5. Hugo says that 8000 mm represent 8 m. Is he right? Explain your answer.
6. Braden writes this equality on the board: $5\mathrm{m}=50\mathrm{d}\mathrm{m}=500\mathrm{c}\mathrm{m}=5000\mathrm{m}\mathrm{m}$. What do you notice?

Source: translated from Les mathématiques...un peu, beaucoup, à la folie!, Guide pédagogique, Numération et sens du nombre 4^e, Module 2, série 2, p. 206.

Activity 2: Addition of Decimal Numbers

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Perform addition using a mental math strategy:

$\begin{array}{l}1.69.7+24.5=\\ 2.61.7+34.3=\\ 3.22.1+63.4=\\ 4.17.9+68.7=\\ 5.193.6+357.5=\end{array}$