Trick for subtracting integers

I have algebra students, in high school and college, who struggle with evaluating expressions like 2 – 5.  This is a ubiquitous problem.

2-5 problem

I have tried several strategies and the one that is easily the most effective is shown below. When a student is stuck on 2 – 5 the following routine plays out like this consistently.

  • Me: “What is 5 – 2?”
  • Student pauses for a moment, “3”
  • Me: “So what is 2 – 5?”
  • Student pauses, “-3?”
  • Me: Yes!

5-2

I implemented this approach because it ties into their prior knowledge of 5 – 2. It also prompts them to analyze the situation – do some thinking.

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