# Double-Digit Subtraction Lesson Plan

#### OVERVIEW & PURPOSE

Week five of the 2nd Grade CountFast program focuses the general strategies for subtracting double-digit numbers with quick mental calculation. Â (Subtraction with â€˜borrowingâ€™ or â€˜trading inâ€™, and subtraction with no borrowing.) Â Card decks should go home with students each day for additional practice with a parent at home. Each week, a new deck is introduced and the previous deck is for the student to keep at home for continued practice.

#### EDUCATION STANDARDS

1. NCTM Standard: develop fluency with basic number combinations for addition and subtraction
2. NCTM Standard: understand the effects of adding and subtracting whole numbers
3. CCSS.MATH.CONTENT.2.NBT.B.5
Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.

#### OBJECTIVES

1. Develop strategies for fluency in solving double-digit subtraction equations by understanding the process of â€˜borrowingâ€™ or â€˜trading inâ€™ when needed.

#### MATERIALS NEEDED

One Double-Digit Subtraction card deck for each student. Â This deck is for school and home use. Â Discuss routine and expectations for taking home the deck and returning it to school each day.

Double-Digit Subtraction – Day 1

Teacher Model/Direct: Â Introduce students to this weekâ€™s card deck. Â Today you will work with the yellow cards in the pack. Â The problems on the cards are written horizontally. Â Discuss/demonstrate with students how these same problems are written vertically, lining up the ones and tens columns as learned in 1stÂ Grade. Â Review the process of subtracting the oneâ€™s column first, and then subtracting the tenâ€™s column. Â When looking at the horizontal representation of the problems (on the cards), show students that they can look quickly at the numbers in the oneâ€™s position of each number to determine if the oneâ€™s will subtract without needing to borrow from the tens group. Â For all of the yellow cards, the oneâ€™s digits calculations do not require any â€˜borrowingâ€™ or â€˜tradingâ€™.

Of course, there are other methods students can use for quick mental calculation. Â Some students may find it faster to decompose the numbers and subtract those smaller parts. (23 – 11 can be thought of as 20 – 10 – 3 – 1, or 20 – 10 -4, or 23 â€“ 10 – 1). Â Encourage students to use the fastest mental calculation method for his/her way of thinking.

Student Activity: Â Partner up students and have each pair use one deck to work with. Â Ask them to take out the yellowÂ cards. Â Students turn over one card at a time and see which partner can solve the problem the fastest, being sure to explain to his/her partner how he/she arrived at the answer.

Home Activity: Students will practice solving the problems on the YELLOW cards with a parent as quickly as possible.

Double-Digit Subtraction – Day 2

Teacher Model/Direct: Â Review the concept of subtracting double-digit numbers when there is no need to â€˜borrowâ€™ or â€˜trade inâ€™. Â Â Today, look at the BLUE set of cards in the deck with students. Â Guide them to notice that on each blue card, the answer when subtracting the digits in the oneâ€™s place equals zero. Â This makes it quick and easy to subtract the numbers in the tenâ€™s place and add a zero on the end to get the final difference. Â For example, 31 â€“ 11 can be solved quickly by noticing that the oneâ€™s digits (1 â€“ 1) will equal 0. Â This just leaves subtraction of the tenâ€™s digits (3 â€“ 1 which equals 2) and then add a 0 after that answer (2) to get 20 as a final difference.

Student Activity: Â Repeat the activity from Day 1, using the BLUE cards in the deck. Â Students should encourage partners to speak through the process to each other and celebrate each otherâ€™s successes.

Home Activity: Students will practice quick mental calculation of the blue cards with parents.

Double-Digit Subtraction – Day 3

Teacher Model/Direct: Â Review the strategies learned on Days 1 and 2. Â Today, use the GREEN/GOLD cards in the deck to quickly subtract numbers when there is a need to â€˜borrowâ€™ or â€˜trade inâ€™ to get the answer. Re-teach what it looks like when the problems are written vertically instead of horizontally the way they are on the cards. Â Review the subtraction concept of the top (or first) number being how much we HAVE to start with. Â The bottom (or second) number is what we are taking AWAY. Â Sometimes there are not enough ones in the top (or first) number to subtract the bottom (or second) number. Â AND â€“ we know that Commutative Property does not work for subtraction, so we cannot just transpose the numbers to make it â€˜easierâ€™. Â Review borrowing or trading in from the tens group to look at the original number differently without changing its value. (42 â€“ 29 requires a trade-in because we cannot take 9 ones away from 2 ones. Â We must rewrite 42 as 3 tens and 12 ones in order to subtract.) Â This lesson may take three days to fully teach so that students can do this process mentally.

Some students will look at the problems differently (such as 42 â€“ 29 can be solved by subtracting 20 from 42, then subtracting 9 more; OR subtracting 30 from 42, then adding one back on). Â Encourage students to use a method that is FASTEST for the way he/she thinks.

Student Activity: Â Partner students and have them use the GREEN/GOLD cards from the deck to take turns quickly mentally subtracting double-digit numbers using â€˜trading inâ€™ or â€˜decomposingâ€™. Â Students should encourage each other to speak through the process used and to celebrate each otherâ€™s successes.

Home Activity: Students will practice quick mental calculation of the GREEN/GOLD cards with parents.

Double-Digit Subtraction â€“ Days 4 and 5

For days 4 and 5, review the three mental calculation strategies and practice with each of the color sets. Â Students can even mix up the color sets, and explain which strategy was used to solve each problem as they compete with their partners. Â Students will do the same at home with parents on these days. Â You may wish to use these days to continue working with the concept of trading in.

For an added challenge, students can play a bracket-style tournament in which all of the yellow, blue, and green/gold cards are mixed together. Â Partners can turn one card over at a time, much like they did with the CountFast 12 game, and see who â€˜winsâ€™ each card by solving the problem the fastest. Â Winners move on to the next round/opponent and those eliminated should cheer on those still competing. Â Celebrate together the accomplishments of this challenging week!

#### Instructional video

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