Math Mammoth Add & Subtract 1

79 pages
(includes answers)

Contents and sample lessons

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Math Mammoth Add & Subtract 1 deals with two major topics from the first grade level: adding and subtracting within 0-20 and adding and subtracting two-digit numbers, mostly with mental math. These two major topics are blended together in a somewhat spiral manner. The subtopics include:

• Adding a two-digit number and a single-digit number without regrouping (carrying) using mental math (for example, 23 + 4 or 56 + 3).
• Subtracting a one-digit number from a two-digit number without regrouping (borrowing) using mental math (for example, 45 − 3 or 67 − 6).
• Adding and subtracting two-digit numbers in columns (one number under the other) without regrouping.
• Recognizing that sometimes in adding two-digit numbers we need to regroup — to combine ten ones to make a new ten.
  
  In this grade level (first), we approach this concept mainly using visual models. Children are also welcome to use the 100-bead abacus to solve these problems. The standard algorithm where one number is written under the other will be studied in 2nd grade.
• Learning specific mental math strategies for adding and subtracting numbers under 20 (such as 7 + 9 or 15 − 8). We study a trick with nine and eight, adding just one more than a known sum, and using the relationship between addition and subtraction to subtract.

Please note that while several basic strategies are included for adding and subtracting within 0-20, the goal is not to memorize the basic addition where the sum is between 11 and 18, and the corresponding subtraction facts. That will happen in 2nd grade. Right now, the idea is to build the student’s number sense: the ability to manipulate numbers by breaking them apart and composing them in different ways.

Also in second grade, your child or students will learn the traditional paper-and-pencil algorithms of adding and subtracting in columns. They will also learn more mental math. This means they will be able to fluently add and subtract numbers within 0-100. They will also learn to add and subtract three-digit numbers.

Good Mathematical Practices

• Mental math strategies allow your child or students to observe patterns inherent in our base-ten number system. For example, knowing that 8 + 7 = 15 allows one to easily add 28 + 7. Or, there is a similarity in the subtractions 10 − 4, 50 − 4, and 80 − 4. This is what good number sense is all about: being able to recognize these patterns and structures, and to use them to do calculations in one’s head.

• Several lessons in ask the child to write an equation (a calculation) for a situation in a word problem. This is the very beginning stage of modeling with mathematics: applying mathematics to solve problems arising in everyday life.

The followup to this book, Add & Subtract 2-A, covers the basic addition and subtraction facts within 0-18 plus mental math strategies and word problems.

The PDF version of this book can be filled in on a computer, phone, or tablet, using the annotation tools found in many PDF apps. See more.