Lesson 24: Subtraction Word Problems within 20
Lesson Objectives
- Subtract within 20
- Apply TouchPoints on lesser numerals
- Compare number sentences to solve word problems
- Use TouchPoints as a strategy in computation
- Take away groups of objects to subtract
- Subtract using Pictorial TouchPoints
- Count backwards using TouchPoints
- Use number lines, number sentences, and part/whole relationships to explain subtraction
Materials
- Extend Workbook (Page 24)
- Counters or manipulatives
- TouchMath’s Foam Numerals and TouchPoints with or without pictures
- Number lines 0-20
- Linking cubes
- White boards and dry erase markers
Step 1: Warm Up (5 min)
Explain to students that when we say and write subtraction sentences, we use minus to mean take away. Repeat the number sentence: 18 minus 5 = 13. The answer, 13, is the difference. Take out linking cubes to illustrate subtraction. Take out 18 cubes linked and show students. Show the students as you take away 5 cubes. Ask how many cubes you have left (Yes, 13 cubes.). The number sentence is 18 minus 5 = 13. Explain to students that math is around them every day and that you will need their help solving story problems. Provide a real-life example (e.g. Mr. Hill is going on a hike. He has 6 hiking sticks to share to give to his 5 friends joining him. How many hiking sticks does he have left for himself? The answer is one hiking stick is left). Dissect sample world problems by circling or highlighting key words in the word problem, and use manipulatives to help count the differences.
Step 2: Vocab Review (5 min)
Review prior vocabulary: subtraction, difference, subtraction, minus, part, subtract, take away, and whole. Introduce the word “left” and explain that when this word is used you know that the operation in the word problem is subtraction. Remind students that the difference is the answer to a subtraction sentence. When you take away one part, you have one part left. It is called the difference.
Step 3: Model (5 min)
Provide another sample word problem with the subtraction equation to the right of the word problem. Show students a good method for breaking down the word problem is to look the key words (eg. left) to show it is a subtraction problem. Next, highlight the first number (e.g. 12 cookies) and highlight the number 12 in the subtraction problem the same color (red), then highlight the second number (7 were eaten) and the number 7 in the addition problem another color (blue). This will help them visually see the numbers in the word problem and matching it to the subtraction problem. Then, highlight the subtraction sign and the subtraction key words (left) the same color (green). Next, solve the subtraction problem, reminding students they already to cross out the first number in the subtraction problem, as they say the number, then count backward on the TouchPoints of the other minuend to find the difference. Remind students when working with the Foam Numerals, to remove the TouchPoints from the minuend when counting backward, as they are “taking away” that number, and also to rememeber to repeat the math fact (12-7=5). Show a few examples using both the counters and Foam Numerals and TouchPoints.
Step 4: Guided Practice (5 min)
After illustrating how to break down addition problems in the modeling section, have students pull out manipulatives (TouchMath’s Foam Numerals and TouchPoints) to assist with counting backward. Students can also group two sets of counters to represent the two numbers in the word problem to then subtract the smaller number from the minuend once students have removed the smaller number counters. Give students subtraction word problems problems with differences no larger than 10 at first, and then work your way up to differences no larger than 20. You can also teach the CUBE strategy when breaking down a word problem (circle important numbers, underline the question, box operation clues, evaluate – determine what steps complete, and then solve and check). Providing checklists for students when breaking down word problems is another great strategy.
Step 5: Student Practice (5 min)
Go to Student Workbook Page (24). Read the directions at the top of the page. Tell students they will be solving subtraction word problems within 20. For the first problem, read the problem to the students. Break down with the CUBE method (circling 12 and 6), underlining the question, boxing key words (left), and then reminding students that evaulate in this case would mean to highlight the first number in the word problem and the minuend in the subtraction problem the same color, then highlight the second number in the word problem and the second number in the subtraction problem a different color. This will help students to visually see the correlation. Then, have students work through the subtraction problem (12-6 by crossing off the minuend, 12, while saying it aloud, then counting backwards on the TouchPoints of the other number (11,10,9,8,7,6), and then repeating the math fact aloud (12-6=6). Write the difference on the line provided to the right of the equation sign. Follow these stueps for the next 5 word problems, using resources, as needed.
Step 6: Wrap Up (5 min)
To wrap up the lesson, review the learning objectives and core vocabulary words again and ask your students about their experience. Circle back to those real-life connections and experiences.
