The purpose of this activity is for students to work with products of whole numbers and multiples of 10 in a concrete and familiar context before reasoning more abstractly about them. Given some numbers of dollar bills (for instance, four $20 bills), students write expressions to represent the amount ( ) and then find its value using strategies that make sense to them. For example, students may count by 20 four times or think of $20 in terms of two $10 bills and find (or ). Consider giving students access to play money, if available, to help them visualize the quantities and support their reasoning.

The reasoning here prompts students to use strategies based on place value and properties of operations (especially the associative property). It prepares students to work more flexibly with products involving factors and multiples of 10 in which the product is greater than 100.

• Invite students to share different combinations of the same bill that could be used to make $150. Record and display expressions for each combination.
• Select previously identified students to share their strategies for how they found one of the totals in the last problem.

The purpose of this activity is for students to continue to reason about products of a whole number and a multiple of 10, this time using base-ten blocks or base-ten diagrams to support their thinking. Students analyze two strategies for multiplying. Both strategies are based on place value, but the second strategy also uses the associative property to think about as or .

• Select 2–3 students who used a strategy based on the associative property (for example, thinking of as 28 tens) to share their responses.
• Consider asking:
  • “Where do we see the original expression in _____’s work?”
  • “How did _____ change the original expression to make it easier to find the total?”
  • “How does _____’s strategy for multiplying work?”