This lesson is preparation for later lessons on solving two-step word problems that require that students translate descriptions of problem situations in words into addition statements of two kinds: those that have a missing sum and those that have a missing addend.

In this lesson, students develop fluency in solving problems based on a missing addend. Also, additional terminology is introduced. Students learn that the term “sum” is the result you get when you add two numbers and that the term “difference” is the result you get when you subtract two numbers.

In the problems in this lesson, students rewrite addition statements with a missing addend as a subtraction statement with a missing difference.

In this lesson, students learn that for any two numbers, there are two addition statements that can be written – one involving a missing sum and one involving a missing addend (or difference). This is the basis for all the two-step problems in this series.

In the problems in this lesson, students solve two kinds of problems:

- They’re given an addition statement with the sum given and a missing addend. They find the missing addend, then use it to solve a second missing addend problem in which the difference is unknown.
- They’re given an addition statement with the difference given and a missing addend. They find the missing addend, then they use it to solve a second missing addend problem in which the sum is unknown.

In this lesson, students learn to use what they learned in the lesson before to solve word problems based on the sum and difference of the same two numbers. They interpret the problem situation to write two addition sentences to find an unknown difference, as described in type 1 above. They then solve the problem using the same steps used in that lesson.

In solving these problems, they write each of the two equations before solving the problem.

In this lesson, students solve the kind of problems they solved in the lesson before. The only difference is that they don’t write the number sentences. They only write the final answer.

In this lesson, students learn to use what they learned in the second lesson in the series to solve word problems based on the sum and difference of the same two numbers. They interpret the problem situation to write two addition sentences to find an unknown sum, as described in type 2 above. They then solve the problem using the same steps used in that lesson.

In solving these problems, they write each of the two equations before solving the problem.

In this lesson, students solve the kind of problems they solved in the lesson before. The only difference is that they don’t write the number sentences. They only write the final answer.

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