Lesson 23

The purpose of this lesson is for students to notice how the unit of ten can be used to find differences within 20.

In previous lessons, students subtracted from teen numbers in a way that made sense to them. The purpose of this lesson is to encourage students to use the unit of ten to find differences. In the first activity, students play the same subtraction game they played in the previous lesson, this time using 10-frames to represent their starting number. By using 10-frames, students can visually make sense of how to use 10 in order to take away efficiently. In the second activity, students analyze taking away to make a ten as a method to find the difference between two numbers.

Although many students may use math tools to help them find the difference, they may choose to write equations to represent their thinking. At this point in the year, students are not expected to write equations that match all of their steps, but teachers should always write accurate equations. Student equations may be accurate, like \(13 - 3 - 3 = \boxed{7}\) or \(8 + 2 + 5 = \boxed{15}\) or inaccurate, like \(8 + 2 = 10 + 5 = \boxed{15}\) or \(13 - 3 = 10 - 3 = \boxed{7}\).

In addition to writing equations, it may also be helpful to represent how the 10 was used in order to find the difference. 

• Each group of 2 needs a set of Number Cards 0 - 10 and a set of Number Cards 11-20 used in a previous lesson.

Check-in with your norms and routines. Are they promoting engagement from all of your students? Are there any adjustments you might make so that all students do math tomorrow?