Days 42 and 43: Careless mistakes with decimals; exponent reflection

Observations on the careless mistakes students make, plus how I have been careless too.

180 days of math post-its

We’re heading into decimals as integers. I want to make sure the students know place value so I gave the standard classes a pre-test on adding and subtracting positive decimals. Before the pre-test, I talked about attending to precision and I specifically asked the students to rewrite the problem vertically to avoid careless mistakes.

Students shared their work on the whiteboard and we talked about each problem.  Of course I had a few who did not take my suggestion to heart. I didn’t have my smartphone handy to capture the actual student work, but below is a recreation of a mistake one student made with the first problem.

decimal mistake

This was a great mistake because it generated a conversation about place value, but I was soooo disappointed the student didn’t rewrite the problem to solve. Perhaps the problem was too easy so they didn’t feel it was necessary to rewrite it…

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