Warm-up: Notice and Wonder: Same Solution (10 minutes)
- Groups of 2
- Display the image.
- “What do you notice? What do you wonder?”
- 1 minute: quiet think time
- “Discuss your thinking with your partner.”
- 1 minute: partner discussion
- Share and record responses.
What do you notice? What do you wonder?
- “Why do you think the results of the two calculations are the same?” (The factors are the same, just in a different order. The order of the factors does not change the result of multiplication.)
- "Which way would you prefer to find the value of \(417 \times 28\)?" (I like the one with two partial products as there is less adding up to do.)
- "Today you will get to choose how to find products of a 3-digit number and a 2-digit number.
Activity 1: Greatest Product (15 minutes)
Students use their understanding of place value to generate expressions that have the greatest product. They take turns selecting number cards 0 through 9 and place them strategically to make the largest product of a 2-digit number and a 3-digit number. Students will need to think about how the different place values influence the value of the product and choose where to put their digits accordingly. There is also a large element of chance since they do not know in advance which numbers they will select.
Supports accessibility for: Visual-Spatial Processing, Organization
Materials to Copy
- Number Cards (0-10)
- Greatest Product
- Groups of 2
- Give each group a set of number cards and 2 copies of the blackline master.
- “Remove the cards that show 10 and set them aside.”
- “We’re going to play a game called Greatest Product. Let’s read through the directions and play one round together.”
- Read through the directions with the class and play a round against the class using the diagram in the student workbook:
- Display each number card.
- Think through your choices aloud.
- Record your move and score for all to see.
- “Now, play the game with your partner.”
- 8–10 minutes: partner work time
- Partner A chooses a number card and writes the number in one of the blanks for Round 1.
- Partner B does the same.
- Repeat until each partner has a two-digit by three-digit multiplication problem.
- Find the product.
- The partner with the greater product wins a point.
- The partner with the most points after 5 rounds wins the game.
- Display a blank image from game board.
- “If this is your game board and you pick a 8, where would you write the 8? Why?” (I would either put it in the hundreds place of the top number or the tens place of the bottom number because it’s a big number and those are the biggest place values.)
- Write the 8 in the hundreds place of the top number.
- “What if you next select a 1? Where would you write the 1? Why?” (I would put it in the ones place of one of the numbers because I want to put a bigger number in the tens place.)
- “What did you find challenging about the game?” (Since I didn’t know what numbers I was going to get on later picks, I sometimes wasn’t sure where to put a number because I didn’t know if I would get a bigger number on a later pick.)
Activity 2: Desperately Seeking 9 New Units [OPTIONAL] (20 minutes)
- they may calculate \(999 \times 9\) and see that while they get very close to composing 9 new units, they fall 1 short
- the biggest product that can be made from multiplying a pair of 1-digit numbers is 81, which would mean that 8 units are composed
- this 81 has to be combined with whatever new units were composed before, but since that’s 8, that means the largest number you can form at each step in the standard algorithm is 89, which is 1 short of the 90 you would need to compose 9 new units
Advances: Speaking, Conversing
- Groups of 2
- 1–2 minutes: quiet think time
- 3–5 minutes: partner work time
Tyler notices that when he uses the standard algorithm and composes a new unit, sometimes there is 1 new unit, sometimes 2, all the way up to 8. He has not seen an example with 9 of the new unit.
For each of these products, how many of each new unit do you compose?
- \(256 \times 5\)
- \(587 \times 8\)
- \(809 \times 9\)
- Do you think it is possible to compose 9 of a new unit with the standard multiplication algorithm?
Advancing Student Thinking
- Invite students to share the number of units they composed in the calculations of problem 1.
- “Was anything missing from 1 to 8?” (No, we composed everything from 1 to 8 new units.)
- “Was anyone able to compose 9 new tens in a product? Why?” (No. The new tens come when I take the product of the ones. The biggest that can be is \(9 \times 9\) which gives me 8 tens.)
- “Was anyone able to compose 9 new hundreds? Why?” (No. I could get 8 new hundreds if I have \(9 \times 90\), giving me 810. I might also have to combine that with the new tens from the product of the ones, but that would give me at most 890, not 900.)
“Today we used the standard algorithm to find products of numbers with no restriction on the number of newly composed units and we examined how many new units can be composed.”
“Do you think the algorithm for multiplying whole numbers will work for any and all whole numbers? Why or why not? Discuss with a partner.” (I think so but if the numbers are big it will take up a lot of space and there could be a lot more new units to compose. I think it might work but it would take a long time if the numbers are big and I think it would be easy to make a mistake.)
Ask students to share their thinking.
“What do you still wonder about the standard algorithm for multiplying whole numbers?” (When is it a good strategy to use? Are there other ways that work well or better?)