“But Mom! I did the math!”

So a slight disaster struck our household recently.  (and I mean slight; this is filtered through the lens of a child!

My kids were recently at my husband's place of work (a large high school),  waiting for him to finish up and drive them home. They kids are used to hanging around there and are often trying to find ways to amuse themselves while they wait for dad.  My oldest son had his reaction ball with him and they decided to play catch with it in the open foyer of the school.  

Reaction balls. The one in question from this story is yellow.

In the usual brotherly fashion, they started to argue about who gets the ball.  One of them (he asked to remain nameless!), threw the ball, hitting the trophy case in the foyer.  The ball is deceptively heavy (about 272 g; a tennis ball is about 58 g);  it ended up cracking the glass of the trophy case!  The child who threw it says it hit the frame of the case three sections down from where the crack is; he’s floating a theory that the crack was already there. He’s also playing with the idea that the force of the hit on the frame sent vibrations throughout the whole case, causing the crack so far from the point of impact.  #science #physics

You can imagine how upset the kids were.  They are really empathetic, kind, never any trouble at school and generally well behaved (A biased opinion, I know!  But I’m their mom and #1 fan)

When I arrived home and heard what went down, I encountered a very sheepish looking older brother and a very sad little brother who sent himself to his room.  

​I went to have a chat with him and he was not in the mood to be cheered up.  He told me that is was going to take more than 100 years to pay off the damage they had done.  I tried to assure him it would not take that long and he said:

"But Mom! I did the math"

Here’s how the conversation went:

Child:  “My allowance is $10 every two weeks so that is only $120...wait $240 a year.”

Me (in my head):  Actually you are assuming that you get your allowance only 2 times a month, some months you get it 3 times.  You are using a bi-monthly calculation, not bi-weekly.

Me (to my son):  awww, honey :(

Son (through tears): “and the glass is going to cost $30 000 dollars so that means like 100 years!!!!  I’ll never be able to get that much money!!”

Me (in my head):  Well, you are assuming that your earning potential for the next 100 years is going to stay the same.  As your mother, I am hoping that you will have a job at some point that pays more that $10 every two weeks.  Also, how much do you think glass costs??? Hmmmm… 30 000 divided by 240 is 125. That’s a pretty good estimate using these assumptions.  My child is a genius.

Me (to my son): “How big was the glass?  It can’t possibly cost $30 000!”

(30 minutes of debating the cost of glass and listening to his various mental calculations)

He went on to explain to me how he did his calculations and after a lengthy discussion, I finally convinced him that 1) the glass did not cost that much, 2) he was not going to be spending the next 100 years paying for this glass, and 3) he was most likely not the cause of the small crack in the glass.  

This conversation got me thinking about all of the little mathematical conversations parents have with their kids.  I know my own kids are experts at negotiating timelines for bed, justifying how much screen time they should have and estimating how long it takes to get out the door for activities (factoring in travel time, and whether they are going to a game or a practice).  

As a math teacher, I notice and capitalize on these moments.  My kids would argue I notice this too much. Sometimes when the kids ask a question that could be reasoned through mathematically, they preface the question with a “I just want the answer - don’t talk to me about the math!!!”.  

If you are looking to create these kinds of moments with your students or supporting their parents in having these kinds of conversations, check out and share the following websites: 

• Talking Math with Your Kids
• Bedtime Math
• This blog post from denisegaskins.com​​

As per my kid’s request, I’m working on not asking too many questions.

Do you know of any other great resources like these?  Let me know!

K.