Math is Figure-Out-Able!

#MathStratChat - March 19, 2025

• Pam Harris, Kim Montague

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In today’s MathStratChat, Pam and Kim discuss the MathStratChat problem shared on social media on March 19, 2025. 


Note: It’s more fun if you try to solve the problem, share it on social media, comment on others strategies, before you listen to Pam and Kim’s strategies.


Check out #MathStratChat on your favorite social media site and join in the conversation.

Twitter: @PWHarris

Instagram: Pam Harris_math

Facebook: Pam Harris, author, mathematics education


Want more? Check out the archive of all of our #MathStratChat posts!

Pam  00:00

Hey, fellow mathers! Welcome to the podcast where Math is Figure-Out-Able. I'm Pam.

 

Kim  00:06

And I'm Kim.

 

Pam  00:06

And this is a MathStratChat episode where we chat about our math strategies. Every Wednesday evening, I'll throw out a math problem on social media, and people from around the world chat about the strategies they use, and comment on each other's thinking. 

 

Kim  00:18

Alright, so this week, our problem was 38 times 42. How would you solve this problem? Pause. Solve it however you'd like. It is 38 times 42.

 

Pam  00:31

So, Kim, you might be guessing what I want to do. Do you want to do what I want to do? Or (unclear). 

 

Kim  00:35

No, I want to do my favorite today. (unclear).

 

Pam  00:38

Okay, you do your favorite. Alright.

 

Kim  00:39

I'm going to do. I should probably stop saying that. I don't know if it's my favorite favorite. I just love it. I'm going to go with 40 times 42.

 

Pam  00:48

Okay.

 

Kim  00:50

And when I think of fourty 42s, I'm actually thinking about double double. So, I know two 42s is 84. So, four 42s is 168. But then it's times 10 because it's 40 and not 4.

 

Pam  01:11

Oh, I like it!

 

Kim  01:14

Okay, so that's 1680.

 

Pam  01:15

Hey, before you go on, can I just think about four 40s is 1,600. And two 40s is 80. Sure enough. Okay, yep, yep.

 

Kim  01:33

Okay, so then that's fourty 42s, so that's too much by two 42s. And two 42s is 84. So, now I have 1680 minus 84. So, back 80, back 4. That is 1596.

 

Pam  01:46

Ha, I wrote 1796. Yep, 1596 Excellent. Well done. Alright, 38 times 42. Both numbers are 2 away from 40. 

 

Kim  01:52

Okay. 

 

Pam  01:52

So, I'm going to rewrite the 38 as 40 minus 2, and the 42 as 40 plus 2, and then I'm going to multiply all those parts together. So, 40 times 40 is 1,600, and I've got a -2 times 40 and a positive 2 times 40. Those add to 0. And then a -2 times a -2 is a - 4. And 1,600 minus 4 is 1596. Bam.

 

Kim  02:15

When you think about this strategy, do you always write all 4 parts?

 

Pam  02:20

I do. Should I not?

 

Kim  02:24

I don't know that "should" is... I just, I wonder if with more experience if there's a generalization that if you're going to add something, and then subtract something, that you can just worry about some of the parts. 

 

Pam  02:45

That's interesting because I've worked so hard to get myself from not just doing a^2 minus b^2.

 

Kim  02:51

Well, then it's good that you're thinking through all of the parts. Alright. Well, we can't wait to see what you all do every single week. Join us on MathStratChat, and let us know how you think about the problems and comment on each other's strategies. 

 

Pam  03:06

I'm still thinking about what you said, but I'll get down to my script in just a minute here. We post the problems on Wednesday at 7:00 pm Central time. When you answer, tag me and use the hashtag MathStratChat. Then join us here to hear how we're thinking about the problem. We love having you as part of the Math is Figure-Out-Able movement! Math is Figure-Out-Able!