Alternative Algorithms

A student who recently came into our district from Australia shared this method of subtraction with me this morning when I was working with her in class. It was new to me, but it works. Unfortunately she was unable to explain the “why” behind it, which is what made me question it at first.

What I’m wondering is how my readers approach student-created algorithms in class. What is your criteria for accepting student-created algorithms? Do you encourage the use of alternative algorithms?

UPDATE: Here is a site that addresses and shows the justification for the Austrian method of subtraction shown above:

About gpsmathcoach

I am an Elementary Math Coach for the Greenwich Public Schools in Greenwich, Connecticut. I serve 11 elementary schools and approximately 240 teachers.
This entry was posted in Best Practices, Everyday Math, Teaching, Thinking/Cognition and tagged . Bookmark the permalink.

2 Responses to Alternative Algorithms

  1. Dave Radcliffe says:

    I have seen this method before. Wikipedia calls it the Austrian method. I would accept and encourage this algorithm. It is possible that the student understands why it works, but lacks the verbal sophistication to explain it.

    • gpsmathcoach says:

      Thanks for the information, Dave. I will definitely check it out. The student is a fourth grader, so it is entirely possible that she just didn’t have a way to explain the algorithm, however I’m almost of the mindset that she was taught the algorithm without the “backstory”, which unfortunately happens far too often.

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