Unit C

How Are Addition and Subtraction Related?

In our last math unit, Unit B, students were exposed to strategies for addition and subtraction. Unit C will focus on using their understanding of the strategies and applying them solve problems with the unknown in all places.

Below are just a few examples of word problems with the unknown in different places.

  • Zak had some crayons on his desk. His sister gave him five more, then he had 11. How many crayons did he start with on his desk?    In this problem, I don’t know how many crayons Zak started with. What I start with is UNKNOWN. ______ + 5 = 11
  • Mrs. Henson has 17 students in her classroom. Some students went to the media center.  Now she has 11 students in the classroom.  How many students went to the media center?  In this problem, I don’t know what CHANGED.            17 – _____ = 11
  • Sydney has 5 horses. Hailey has 12 horses. How many fewer horses does Sydney have than Hailey?  In this problem, I  don’t know the RESULT.  With this problem, I could subtract 12 – 5 = _____.   So, I could use addition and think    5 + _____ = 12

When we look for unknowns, it is important for us to use what we know about the relationship between addition and subtraction.

In this unit we will also explore the EQUAL symbol/sign and what it means.  Many students think that the equal sign means: “the answer is”.  We will soon discover that we can replace the symbol with the words “is the same as” or “has the same value as”.


We will practice working with the sum or difference being presented to the left of the equal sign.

5 + 6 = 11  or 11 = 5 + 6



AND we will work with related facts/fact families