B2.3 Use mental math strategies, including estimation, to add and subtract whole numbers that add up to no more than 1000, and explain the strategies used.
Activity 1: Using Mental Math Strategies to Add Numbers
Mentally calculate the cost of the following 3 items.
Invite students to share their strategies with their classmates. Encourage them to use and explore a variety of manipulatives (for example, the number line) to effectively express and represent their mathematical thinking.
Activity 2: Using Mental Math Strategies to Subtract Numbers
Mentally solve the following:
\(845 - 50 =\)
\(903 - 146 =\)
\(598 - 391 =\)
\(758 - 231 =\)
Ask students the following questions:
- How do you round numbers? To the nearest ten? To the nearest hundred? How do you know? Justify your answer.
- What strategies can be used to mentally solve these subtraction questions? Explain your choice.