# Tape Diagrams: Modeling 2-Digit Addition and Subtraction

Your child continues to use tape diagrams as a visual tool to solve word problems. Now, he solves for an unknown in any position.

Here’s a second grade word problem: There are twenty-three students in Miss Nord’s class. Seventeen students are present today. How many students are absent?

Let’s draw a tape diagram to see what this problem asks us to find. We know there are twenty-three total students. This represents the twenty-three students in the whole class. The seventeen students in class today are represented by this part.

Look! We know our whole, and we have one part. This is what we need to find. One missing part tells us we have to subtract! Now we can write the number sentence. Your child knows exactly what he is solving for and how to solve it.

Let’s use our counting on strategy . Seventeen, eighteen, nineteen, twenty, twenty-one, twenty-two, twenty-three. Six! Our missing part is six.

Twenty-three students take away seventeen students equals six students. Now we know that six students were absent today.

What if the problem tells us seventeen students are present and six are absent. Then, we need to find the whole. We know both parts, so we find the whole by adding. Seventeen plus six equals twenty-three.

Keep practicing and your child will recognize: When we know both parts, we add. When we are missing a part, we subtract. Your child will continue using tape diagrams to figure out how to solve problems third grade but with larger numbers!

And that’s good to know.

Common Core Grade 2 Standard Operations & Algebraic Thinking: Represent and solve problems involving addition and subtraction.

(2.OA.1) Use addition and subtraction within 100 to solve one- and two-step word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g., by using drawings and equations with a symbol for the unknown number to represent the problem.