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    • ▾1st grade
      • ▸Place value
        • •Count and write numbers within 120
        • •Skip count by 3s and 4s
        • •Place value with tens
        • •Compare two-digit numbers
      • ▾Addition and subtraction
        • •River crossing puzzles
        • •Add and subtract within 20
        • •Add within 20, word problems involving three addends
        • •Use one addition problem to solve another
        • •Add by making tens
        • •Subtract by adding
        • •Understand the equals sign
        • •Add and subtract within 20, missing number problems
        • •Add and subtract within 20, word problems
        • •Add within 100, concrete and pictorial
        • •10 more or 10 less
        • •Subtract multiples of ten from multiples of ten
      • ▸Measurement, data, and geometry
        • •Order without tools
        • •Measure given a unit
        • •Bar and picture graphs having three categories of data
        • •Tell and write time, hour and half-hours
        • •Definitions of shapes
        • •Compose 2D and 3D shapes
        • •Halves and quarters
     › 1st grade › Addition and subtraction

    Add and subtract within 20

    First, students will learn how to add within 20. One method, is to add by counting forwards, starting from the first addend. Another method, is to add by making tens. For example, 5 + 9 = 4 + 10 = 14.

    Watch these Khan Academy videos:

    • Adding within 20 using place value blocks
    • Adding within 20 using ten frames
    • Adding 7 + 6
    • Adding 8 + 7

    Do these Khan Academy exercises:

    • Add within 20 visually
    • Add within 20

    Next, students will learn how to subtract within 20. One method, is to subtract by counting backwards, starting from the minuend. Another method, is to subtract by making tens. For example, 13 - 4 = 10 - 1 = 9.

    Watch these Khan Academy videos:

    • Subtract within 20 using a number line
    • Subtract within 20 using place value blocks
    • Subtract within 20 using ten frames
    • Subtracting 14 - 6

    Do these Khan Academy exercises:

    • Subtract within 20 visually
    • Subtract within 20

    This standard is too vague. Exactly which methods should they be using? For example, can they add and subtract with a number line, or is that not considered fluent?

    Conclude by giving this challenge. A couple additional questions can be found here.

    Next, give your students these challenges:

    • 2017 Math Kangaroo Levels 1-2 Problem #18 by STEM4all

    Make the equation below true by replacing each letter with a unique digit (0-9).

    $$W + O = OF$$

    Here's the solution:

    The largest sum of two digits is \(9 + 9 = 18,\) so \(O = 1.\) Now we have \(W + 1 = 1F.\) The only way to make \(W + 1 \ge 10\) is if \(W = 9.\) Now we have \(9 + 1 = 1F,\) so \(F = 0.\) In conclusion, the equation is \(9 + 1 = 10.\) Note that it doesn't matter whether we accept leading zeros, such as \(05,\) because if \(O = 0,\) then \(W = F,\) which violates the condition that \(W\) and \(F\) must be unique.

    Conclude by leading this investigation:

    Raindrops Down Windshield (number order, algorithm)
    by MathPickle

    1.OA.C.6: Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., 8 + 6 = 8 + 2 + 4 = 10 + 4 = 14); decomposing a number leading to a ten (e.g., 13 - 4 = 13 - 3 - 1 = 10 - 1 = 9); using the relationship between addition and subtraction (e.g., knowing that 8 + 4 = 12, one knows 12 - 8 = 4); and creating equivalent but easier or known sums (e.g., adding 6 + 7 by creating the known equivalent 6 + 6 + 1 = 12 + 1 = 13).

    1.OA.C.5: Relate counting to addition and subtraction (e.g., by counting on 2 to add 2).

    Additional lessons and practice problems