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
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 \(6\; \times \;1000\) 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: translated from Les mathématiques...un peu, beaucoup, à la folie!, Guide pédagogique, Numération et sens du nombre 4e, Module 2, série 2, p. 206.
Activity 2: Addition of Decimal Numbers
Perform addition using a mental math strategy:
\(\begin{array}{l}1.\;69.7\; + \;24.5\; = \\2.\;61.7\; + \;34.3\; = \\3.\;22.1\; + \;63.4\; = \;\\4.\;17.9\; + \;68.7\; = \\5.\;193.6\; + \;357.5\; = \end{array}\)