This lesson plan will explore various methods for subtracting tens and ones, with a focus on understanding place value through number lines, hundred squares, and Gattegno charts. We'll emphasize how known facts, rather than counting back, can help your learner subtract efficiently.
Here are a few concepts that are helpful to know for this lesson:
Preparing for Two-Digit Subtraction: When your learner becomes comfortable subtracting tens and ones from a two-digit number, they develop a foundation for more complex subtraction problems. For example, solving 45 - 23 can be approached by first subtracting the tens (20) and then the ones (3).
Base-Ten Blocks: In this lesson, dienes blocks (base-ten blocks) can be used to represent the minuend (starting number) in subtraction, contrasting with addition where both numbers are represented. For example, to subtract 45 - 20, start with 45 base-ten blocks and remove 20 (2 tens).
First-Then-Then-Now Stories: These stories follow a logical sequence, helping learners understand how subtraction can occur in steps. For example, a story might start with, “At first I had 45 biscuits. Then I sold 20 biscuits, and then I dropped 3 biscuits. How many are left now?”
Teaching Plan
The following activities will help your learner become confident with subtracting tens and ones.
Examples and visuals to support the lesson:
1. Review of Subtraction
Start by reviewing subtraction of a one-digit number from a two-digit number using number lines and hundred charts.
For example, demonstrate 45 - 3 using a number line by starting at 45 and move three steps back. Or on a one-hundred chart, show 64 - 40 by starting at 64 and moving up 4 rows to represent subtracting 4 tens.
Discuss how subtracting a multiple of ten affects the tens place, while the ones digit remains the same.
Use examples like 50 - 10 or 30 - 5 to show that subtraction can be done using known facts instead of counting back, which builds fluency.
You can also have your learner practice with base-ten blocks. For example, for 45 - 20, use four ten-blocks and five one-blocks, then remove two ten-blocks to show the result.
Explain how only the minuend (the starting number) is represented in subtraction, whereas in addition, both numbers would be represented.
Skill Check
I can use math tools to subtract tens and ones from two-digit numbers.
2. Combined Subtraction (Tens and Ones)
Next, use story problems to introduce subtracting tens and ones consecutively.
Present a story problem like, “At first I had forty-five biscuits. Then I sold twenty biscuits, and then I dropped three biscuits. How many are left?” Use base-ten blocks to represent the steps.
Then, repeat the process but changing the order of subtraction. This time subtract the three biscuits that were dropped, then the 20 biscuits that were sold.
Illustrate the different orders side by side to show that both result in the same final answer.
Skill Check
I can subtract tens and ones from a two-digit number at the same time.
3. Practice with Additional Story Problems
For additional practice, provide a variation of story problems including partitioning and reduction stories. Here are some examples:
“Charlie counted thirty-six mini-beasts on a patch of grass; ten of them were grasshoppers, and four of them were ladybirds; the rest were ants. How many ants were there?” (partitioning)
"There are eighty-four pens in a jar; two of them are red, and thirty of them are black; the rest are blue. How many blue pens are there?" (partitioning)
“Joel had seventy-five cents. He spent five cents on a candy, and then thirty cents on a pencil. How much money does he have now?” (reduction/take-away)
"Pari had a thirty-centimeter length of thread for making a bracelet. She cut off ten centimeters; the thread was still too long so she cut off another two centimeters. How long is the thread now?" (reduction/take-away)
You can also have your learner create their own story problems.
Provide a set of number cards, like 20, 15, 40, 5, and encourage your learner to create their own subtraction stories.
Have them use base-ten blocks or sketches to visualize the subtraction and explain the story.
Skill Check
I can solve word problems that involve subtracting tens and ones from two-digit numbers.
Summary
This lesson plan reinforced various methods of subtracting tens and ones, using tools like number lines, hundred squares, and Gattegno charts. By emphasizing the place value and known subtraction facts, your learner can develop a deeper understanding of subtraction, beyond just counting back. Encourage your learner to practice with story problems and create their own, reinforcing these essential math skills.
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Skill Check