It's Elementary My Dear Watson

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SWBAT identify and build numbers based on base ten language.

Big Idea

Students solve a mystery by figuring out the mystery number using base 10 "clues".


5 minutes


We have been taking large groups of objects and breaking them into groups of ten and some left over. Today we will take what we know about the tens place and the ones place and use it to figure out the mystery numbers!



We are going to be using big numbers in our story problems one day so we need to understand how to break these numbers into groups. Using those groups will help us as we figure out how to add and subtract these groups later in the year.


Objective :

Your thinking job today is: How can I build a number using tens and ones clues?

See attached video on motivating students in this lesson!

Opening Discussion

15 minutes

I'll start by showing a place value model for 64.

 "I have some tens over here. Let’s make sure there are ten in each group. (Choral count). Yes! Each group has ten. I also have some ones leftover. How many ones do I have? (Choral Count)." 

Guiding Questions:

  • Turn and talk with your partner: How could you figure out how many ones there are in this model?
  • Why can’t we count the leftover ones by 10s? Why do we count the groups of 10 by tens?

After leading students in a choral count, we will write the number 64 on chart paper to refer back to throughout the discussion.

  • Where can I find 4 in my model? Where can I find the 6 in my model?


I'll summarize student thinking: "So in the number 64, there are 6 tens, and I see a 6 in the tens place and 6 tens in my model, and 4 ones. I see 4 leftover cubes here and a 4 in the ones place."

Present problem: I have a mystery number for you: 7 tens and 5 ones. What number do I have?


Partner Talk: How can you figure out what number I have?

Most students will just “know” by this point. Push those kids to explain HOW they know. How are they sure the number is 75? Why not 57?

 I'll choose students to share out how they know it is 75, particularly emphasizing why it is not 75. This emphasis on why the answer isn't 57 pushes kids to reach the Mathematical Practice standard, "Construct viable arguments and critique the reasoning of others." This process models to them that they can't say they "just know" the answer-they have to always have a viable reason.



Game Rules and Practice

10 minutes

Game setup: Today we are going to play a game called Mystery Number with a partner to help us practice figuring out how many tens and how many ones are in a number.

Game Rules:

1. Draw a mystery number card.

2. Both partners build the number using cubes.

3. Both partners count and figure out how many cubes there are in all.

4. Both partners record on their sheet.

 Game Practice:

We are going to play a round of Mystery Number together. I am going to draw the card, but you are going to build the number with your partner and figure out what my mystery number is. When you think you know it, you can draw how you made the number with cubes and record how to write the number.


I'll play a few rounds with students to practice. 

Guiding Questions: 

  • What about this clue helps you figure out the number? 
  • How can we count our base 10 model to check and make sure we are right?
  • How many tens and ones are in this number? 


The recording sheet and game cards are attached in the Independent Practice section.

Independent Practice

15 minutes

Students play the game with a partner, recording how they built the numbers for each number card.


Group A: In Need of Intervention

I will play the game with these students on the rug. They will most likely need help with switching how they count, and understanding why they need to switch their counting.

Group B: Right on Track

Students are using ten rods and ones cubes with fluency. Students should be able to switch between counting by 10s and 1s pretty fluidly. Students probably know what number they are making using just the base ten language before they build it.

 Group C: Extension

Students pull 2 cards and figure out the mystery number that these two cards create when you add them together.

Students probably build all the tens and ones. Students may need to be pushed on how to count the tens and ones, as they may not immediately think to group the tens. This activity will prep students for later 1.NBT standards where they are adding within 100.

Mystery Number Cards are attached!

Recording Sheet can be found here.


5 minutes

I'll sum up the day by reviewing the objective and having students review what they learned today.

"Today’s thinking job was: How can I build a number using tens and ones clues?"

 I'll choose 1 student to share what their number clue was and how they figured out the mystery number!