## Chapter 20

### Part 1: Home and School Investigation

Send the Letter to Family (PDF file) home with each child. Once all the children have brought in a picture with a number sentence, put the children in pairs and have them share their drawings with each other. A good time to do this activity is after Lesson 4 on subtracting from 17 through 20.

### Part 2: Be an Investigator

A good time to do this investigation is after Lesson 4 on subtracting from 17 through 20.

#### Materials

- number cube with the numbers 1 through 6 on it for each pair

#### Introducing the Investigation

Tell the children that they are going to start with the number 18 and see how long it takes them to get to zero. Explain that they will write the starting number, 18, on a piece of paper. Then they will roll a number cube and subtract from 18 the number shown on the number cube. They will record the subtraction on a piece of paper. The answer to the subtraction problem is the next starting number. Tell them that they will continue doing this until they get to zero.

You might want to demonstrate how to record. Suppose you rolled the number 5 the first time. Your record would say:

- 18 − 5 = 13

Then suppose you rolled 4. Your record would say:

13 − 4 = 9

Point out that as they get toward the end they will run into some numbers that are too large to subtract. Tell them to just roll again in that case.

#### Doing the Investigation

Observe children as they do the investigation and make sure that they are recording properly. Some children are likely to get confused toward the end of the game if they roll a number that is larger than the number they are to subtract from. For example, suppose they roll a 6 and the start number is 4. They should roll again, but some children are likely to record 4 − 6 = 2. When you see that happening, explain that they cannot change the start number. If the number that is supposed to be subtracted is too large, they should roll again.

Here is an example of what a record sheet might look like:

- 18 − 6 = 12
- 12 − 3 = 9
- 9 − 5 = 4
- 4 − 3 = 1
- 1 − 1 = 0

#### Extending the Investigation

Have the children play the game again with a different starting number.