David Wees @davidwees asks:

"What is the mistake?"

"What is the thinking that led to this mistake?"

I saw both for (b) and (c) but the train of thought in (a) escaped me completely. What do you think happened in (a)?

What would you tell the students in each case?

If you'd like to watch, the video is about 5 minutes long.

For (a), multiply 9 x 5. Write down 5, then carry the 4. *Add the 4 you carried to the top 4* to get 8, then multiply by 5 to get 40.

ReplyDeleteMultiply 9 x 2. Write down 8, then carry the 1. *Add the 1 you carried to the top 4* to get 5, then multiply by 2 to get 10.

I've taught 4th and 5th a long time. I saw the error pattern in all three right away. That's what happens when you teach procedures.

ReplyDeleteA slight variation on possible thinking compared to Jason's. (They are equivalent but a 4th grader wouldn't necessarily know that)

ReplyDeleteMultiply 9*5 then carry the 4. Multiply the 4 by 5 and the 4 you carried by 5 then add them together.