Samples of noticing and wondering
I notice that the answer is always on the left.
I notice that the first number on the right (minuend) stays the same.
I notice that the number being subtracted (the subtrahend) goes down by 1 each time.
I notice that the answer (difference) goes up by 1 each time.
I notice that the 1 and the 3 trade places in two of the equations.
I wonder why the difference goes up by 1 when the subtrahend goes down by 1.
I wonder what happens to the difference when the minuend goes up by 1.
I wonder if the pattern keeps going if you put more equations into the list.
I wonder if the difference always goes up by the same amount that the subtrahend goes down.
I wonder if you can fit more equations in between the equations in the list.
Creating something new
Students may create and explore their own patterned lists of subtraction equations. Many of their creations may flow out of things that they have wondered about. Examples:
Create lists in which you start with numbers other than 4.
Create lists in which the numbers you are subtracting increase or decrease by different amounts.
Fill new equations in between the equations in your list. (This may involve thinking about fractions / mixed numbers.)
Other ideas: Draw pictures that illustrate some of the patterns that you discover. Create stories to fit your lists of equations.
You can use this type of prompt to help your students understand subtraction more deeply. In this case, recognizing how differences are affected when you change the minuend and/or subtrahend is an important part of making sense of the concept of subtraction. Deeper understanding can also lead to new strategies. For example, students may discover that increasing both the minuend and subtrahend by 1 (or some other amount) leaves the difference unchanged. They could use this idea to make certain subtraction calculations easier: for example, by changing 17 – 9 into 18 – 10.
Some students may wonder what would happen if they continued the list downward (or upward), which may lead to a discussion of negative numbers. The ideas of "how much more" and "counting up" are great ways to think about subtraction with negative numbers (probably more helpful than the "taking away" idea), especially when you visualize the equations on a number line.