Day 27: 9/10 of a cent

Geometry took a concept test today and worked through a few logic puzzles. After Mrs. Murphy’s Laundry they were loving these.

In Algebra I am just starting the wrap up the first chapter. For some reason today’s lesson is another of my favorites. It doesn’t require much but students always go crazy over it.

This year I approached the lesson in a 3-Act sorta way.

I started by throwing this up on the projector:


Students have a short conversation over anything they notice, eventually the 9 comes up. Some students though it was a G for gallons, others had never actually paid attention to it.

“How much did gas cost today?” 

Every. Single. Student. replied with $3.43.

Those students who picked up on the 9/10 of a cent thing quickly self corrected and said $3.44. However, most students didn’t think that 9/10 of a cent even mattered; it added up to less than 20 cents per trip. After a while, we started talking about currency and how there are no coins/bills for tenths of cents.

Students were perplexed at this point; why would companies even do this?

They started talking about how it 9/10 of a cent does actually matter, but only really when you are talking about thousands of gallons of gas.

I chime in with “how much extra each year do you think the gas companies make by charging 9/10 of a cent?” – Gotta strike when you can.

We wrote down guesses, attached names to them, and put them up on the board. After, students thought about what info they would need to solve this problem: How many gallons are sold per year is really the only thing.


They converted a few numbers and did the calculations; it turned out to be in the billions.

Here is my favorite part;

How big is a billion? 1,000,000,000

How many millions are in a billion? Hundred millions? Thousands?

Well I decided to look a little deeper into what a billion actually is. In this word document I have 1 billion represented as groups of 10,000″


Students initial reaction was “No big deal”.

Then I hold the down arrow for about 30 seconds.


After a while students look at the bottom left…



Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s