The purpose of this activity is for students to solve two story problems that highlight the relationship between addition and subtraction. Both are Change Unknown stories that use the same numbers. Although one story “sounds” like addition and the other subtraction, both stories can be solved using either operation. The same equations can be used to solve both problems.

If students choose to write equations to represent each problem, there are many equations they could write. The important thing is for students to be able to explain how the equation they wrote matches the story problem. Some students may write each of their steps as equations.

For example, for students may write:

• Invite previously identified students to share their equation for each problem.
• If needed, ask, “¿Cómo corresponde su ecuación a la forma como resolvieron el problema?” // “How does your equation match how you solved the problem?”
• “¿Qué observan sobre la ecuación que se usó para resolver cada problema?” // “What do you notice about the equation used to solve each problem?” (They could be the same. You can add or subtract for both problems to solve it.)
• “¿Por qué el número desconocido es el mismo en todas las ecuaciones?” // “Why is the unknown number the same in each of these equations?” (Because and . The difference is the same whether I add or subtract.)

The purpose of this activity is for students to solve related addition and subtraction story problems with the unknown in different positions. Students work with a partner to solve a story problem and create a poster of their work. They share their work with groups who solved a different problem and compare their representations and methods.

• Display a list of equations one group made for each problem.
• “¿Qué observan sobre las ecuaciones que este grupo usó en los distintos problemas?” // “What do you notice about the equations this group used for each problem?” (They used the same equations for each problem. All the problems have more than one equation. All the problems have addition and subtraction equations.)