Lesson 25: Subtraction within 99
Lesson Objectives
- Review Touching/Counting Patterns for numbers 1-9
- Apply understanding of place value to subtract within 99 without regrouping
- Use visual cues to aid in subtraction algorithms
- Apply TouchPoints on lesser numerals
- Identify pairs of numbers whose difference is within 99
- Use TouchPoints as a strategy in computation
- Take away groups of objects to subtract
- Subtract using Pictorial TouchPoints
- Count backwards using TouchPoints
Materials
- Extend Workbook (Page 24)
- Counters or manipulatives
- TouchMath’s Foam Numerals with TouchMath’s TouchPoints with or without pictures
- TouchMath’s number line (optional)
Step 1: Warm Up (5 min)
Give students practice subtraction problems within 10 and review strategies from prior lessons: 1) start on the minuend, 2) cross the minuend out while saying it aloud, 3) count on the TouchPoints of the smaller number to find the difference, and 4) repeat the math fact aloud. After reviewing a few problems, show a double-digit subtraction problem (without regrouping) and explain that they will be doing what they just did but now with two-digits.
Step 2: Vocab Review (5 min)
Review prior vocabulary: subtraction, difference, subtraction, minus, part, subtract, take away, and whole. 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. Review the visual cues of the columns to help students “stay in their lane,” and “to start on the side with the arrow in the ones column.”
Step 3: Model (5 min)
When modeling double-digit subtraction without regrouping, remind students to start on the side with the arrow in the ones column, and then go through the usual steps: 1) start with the minuend (larger number), 2) cross the minuend out while saying it aloud, 3) count on the TouchPoints of the smaller number to find the difference, and then 4) say the math fact aloud. Then, have them move on to the left column in the tens place and follow the same steps. Afterwards, repeat the entire problem (e.g. “54-23=31”), as this will help students to practice saying larger numbers aloud.
Step 4: Guided Practice (5 min)
After modeling practice problems with your students, ask students to volunteer to show the correct steps when subtracting double-digit numbers without regrouping. Students can also take turns in pairs or in groups and see if students can get all the steps down without help.
Step 5: Student Practice (5 min)
Go to Student Workbook Page (25). Read the directions at the top of the page. Tell students they will answer the problems to solve the riddle at the bottem. Based on the differences of their answers, find the matching difference at the bottom and write its corresponding letter there. Review the first 1-2 problems with students, reviewing the correct steps: 1) start on the side with the arrow in the ones column, 2) start with the minuend (larger number), 3) cross the minuend out while saying it aloud, 4) count on the TouchPoints of the smaller number to find the difference, and then 5) say the math fact aloud. Repeat these steps in the tens column, and then repeat the entire problem (e.g. “29-20-9”), as this will help students to practice saying larger numbers aloud. Write the letter “T” above the 9 number at the bottom of the page. Work through each problem until students have finished the riddle.
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.
