Five Minutes – Mental Math Monday

Five Minutes – Mental Math Monday

No calculators. No pencils or pens. No fingers. No air writing.

(Gosh, I sound so negative!)

You’ll have some time to think about the multiplication problem on the screen. After a few minutes, I’d like volunteers to share the strategies they used to calculate the answer. Be ready to explain how you arrived at your answer. Right now, your method is more important than your answer. It’s OKAY to be wrong. We will all learn from hearing about different ways to do this multiplication. Everybody ready? Here we go!

I display 5\times 34 on the screen. Everyone starts multiplying in their heads. One or two students get frustrated and reach for paper. I give a gentle reminder to do everything mentally. Some students finish quickly and exclaim “Got it!” I calmly state that there’s no rush to get an answer. After a minute or two, everyone looks ready to go.

Okay, let’s hear some strategies. Remember: I’m not really interested in the answer. I’m excited to hear how you got that answer.

“I knew that 5\times 30 is 150 and that 5\times 4 is 20, so I added them and got 170.”

“I thought 10\times 34=340, and 5 would be half of that. So it should be 160.”

“I did the 10 times thing too, but half of 340 is 170.”

“I did 34+34=68. Then I doubled it to get 136, and I added 34 more to get 170.”

We only have about a minute left, so I project another problem. This time I want the class to calculate 6\times 34.

We’re almost out of time, but before we go, I want you to give this multiplication a try. You might think about using one of the strategies your classmates shared.

My curiosity gets the best of me, and I cut things short to hear some strategies.

“I did 6\times 30=180 and 6\times 4=24 and added them to get 204.”

“Wait! If 5\times 34=170, wouldn’t 6\times 34 just be 204 because you just add another 34.”

Nice work today, everyone. Enjoy the rest of your day!

Five minutes wasn’t enough time for the in-depth discussion I’d like to have. But these are 8th grade students, and I’m not sure how I can justify spending much more time on multiplication when we have so many other areas to work on. Besides, they’ll have calculators on the AIR test…which I don’t actually care about. I want to help every student strengthen his/her number sense. I just don’t know how to fill in all the gaps. I’d love to say that reinforcing multiplication strategies now will pay off in the long run – I know it will! Will it give me the same bang-for-my-buck that working on fractions or graphing or solving equations or so many other topics will?

(Interestingly, I saw these multiplication problems written on a few desks when I cleaned up my classroom at the end of the day. I’m intrigued that some students apparently wanted to get a correct answer so badly on a no-stakes activity.)

I stopped doing Mental Math Monday midway through the year. Mystery Monday took its place, and Get On My Fraction Level! involved some mental computation as well. So what’s the plan for this year? I’m not sure yet. Number sense has been a major area of weakness the past two years; I expect it to be one again this year. Perhaps a combination of these three routines will give me the results I’m looking for, and of course, I will continue looking for new ways to help my students develop their number sense.

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