Beacon Lesson Plan Library

Twenty Is Too Many

Jay Babcock
Colleges and Universities - Florida


Students evaluate and write expressions using subtraction of the number one.


The student knows appropriate methods (for example, concrete materials, mental mathematics, paper and pencil) to solve real-world problems involving addition and subtraction.


-Book: Duke, Kate. [Twenty Is Too Many]. New York: Dutton Books, 2000.
-Manipulatives such as bears, blocks, coins, etc. (at least 10 per student)
-Transparent overhead manipulatives
-Colored chalk
-Number track or number line
-Subtraction worksheet (See Associated File)
-Overhead projector


1. Get a copy of [Twenty Is Too Many] by Kate Duke and familiarize yourself with it.
2. Gather at least 10 manipulatives for each child. They can either be distributed before the lesson, or not until they are needed (step #8). Students needing extra help may need to use them beforehand so they can count along in step #3.
3. Secure for your classroom a device or number line that can illustrate "one less than" subtraction number sentences.
4. Have colored chalk and manipulatives for overhead projector use available.
5. Prepare student copies of the subtraction worksheet. (See Associated File)


1. Read [Twenty Is Too Many] by Kate Duke aloud and show pictures, being careful to point out the subtraction problems.

2. Ask students if they have ever needed to know what “one less than a number is” and to give examples of those times.

3. Using manipulatives, demonstrate by first showing 10 items, and writing “10” on the board. Then, take one away and write “- 1 =” on the board. Then, have students count the remaining items to come up with 9 and write “9” in the appropriate place. Repeat this activity 2 more times.

4. Repeat step #3, but have students come up to write a specific piece of the expression.

5. Have 10 students stand in front of the class. Tell a story that causes students to leave the line one at a time. As each leaves to go back to his seat, he writes (on the chalkboard, overhead, etc.) the equation that just occurred. Repeat, until all students have had a turn.

6. Using the number track, students can see that when the starting number is locked into place and a spacer is inserted onto the track, then the answer slider is pushed into place and points to the correct number. This can be used as a demonstration piece and students can use it for extra assistance in the following activities.

7. Students need paper and pencil. Students record the operations demonstrated by the children in front of the class (during #5). This allows them to practice writing subtraction number sentences. The teacher can circulate to monitor responses or have the page collected.

8. Students need manipulatives, paper and pencil. Students are asked to demonstrate five subtraction problems showing "one less than" with their manipulatives and write them on paper. They should show the teacher each one by raising a hand before moving on to the next.


Formatively assess students on the following:

1. Assign a worksheet with many different problems dealing with the subtraction of the number one. (See Associated File) When asked, students point to the correct starting value, subtracted value and resulting value in a sample problem. (This can be done individually or in a group.)

2. When shown a subtraction problem, students correctly demonstrate both the set-up and solution using manipulatives. For example, in the problem “5 – 1 = ”, the student takes 5 manipulatives, removes one and shows that there are 4 remaining. (This is an individual assessment.)

3. The teacher shows a series of subtraction problems to the class using overhead manipulatives. Students record the problems on a piece of paper using the proper format.


1. By asking students to identify the title and author of the book and then to make predictions as to what the story may be about, the lesson would align with this standard (The student uses titles and illustrations to make oral predictions).
2. Students work on their handwriting skills as they practice writing the numbers 0-9.
3. A very similar lesson could be done for addition using [One Guinea Pig Is Not Enough] by Kate Duke (Dutton Books, 1998).

Web Links

Web supplement for Twenty Is Too Many
A Follow-Up Lesson

Attached Files

This file contains a subtraction worksheet.     File Extension: pdf

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