#MTBoSBlaugust – Day 2
Ah, unit conversions. How many pints are there in a gallon? Cups in a quart? Feet in a mile? These skills are taught, per Common Core, in 5th and 6th grade. The only mention of them in Algebra 1 is in the Number & Quantity section where it states:
Reason quantitatively and use units to solve problems.
Shouldn’t be a huge problem, right? Right? Wrong! It seems every year I spend way too much time teaching and reteaching unit conversions to my students. That almighty decimal point really has the power to mess with their heads. My students are not the only ones who struggle with this concept. I took this photo at a local gas station last month. I don’t think I will be buying much gas here!
My co-teacher loves to use an example from a local gas station. The sign out front said,
Two hotdogs for .1¢
This always leads to wonderful discussions about decimals and unit conversions. Even big companies fall down over unit conversions. Listen to the following clip from a customer’s argument with Verizon over the cost of data used while in Canada.
Do your students hate this concept as much as mine do? How do you handle it?
McLeod's Crowd 
Haha my students are not thrilled about this idea and groan EVERY time I mention the word conversion. In science, it happens often. I try to make it fun by using King Hector Drinks Basically Delicious Chocolate Milk for the conversion of metric system measurements. Beyond that, nothing fun comes from conversions in middle school. I like the idea of using real-life scenarios to help students see why it is so important to understand this concept.
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I love doing “bunny hops” in the metric system!
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I found it interesting that the new Common Core Regents exam reference sheet includes all the basic conversions. They just have to know how to set up the equations to convert if the amount is more than 1.
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I was good at conversions in my 8th grade science and math class, but I was just good at guessing whether to multiply or divide so that I could get a larger or smaller number to fit the unit change. My 9th grade biology teacher really made me understand conversions by using what she called the “diagonals cancel” method. We set everything up based on fractions and made sure that the units would cancel (one on top and one on bottom to form a diagonal line when you crossed it out). That was a magnificent moment for me when I realized WHY I was guessing multiply or divide for each of those problems.
I have tried to teach this to my 9th and 10th grade students (although it fits in science classes better because my curriculum does not include many units), but they do not have enough understanding of fractions from elementary and middle school to see the units cancel each other.
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You make a good point about fraction weakness making this unit overly difficult. I can’t remember which teacher I had that taught me your “diagonals cancel” method. Whoever it was used to chant “times sign draw a line”. You come up with the initial fraction, then times sign draw a line, and use the denominator units in the numerator of the next fraction. I still mutter that under my breath when converting!
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I never thought about drawing the line before writing in the equivalency in that second fraction. Maybe that will help to see where to put the units that you desire to cancel. I usually discourage this because I do not want students to start cancelling just because they THINK it should cancel, but maybe it could be more of a roadmap.
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I swear every year when I review this, my students look at me like I have two heads. They all earnestly tell me they have never seen it before and have no idea what I am talking about. Ugh!
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Well I’m glad my students aren’t the only ones that struggle with this. I have to spend time at the beginning of every school year reviewing this and reminding students that in science we use the dreaded metric system. Though my students do think I’m magic due to my ability to covert metric and standard measurements in my head.
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I’m pretty sure I think you’re magic too! I still struggle with the metric system. You lose something when you don’t use it regularly.
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My students think it is impossible to convert between metric and standard. They cannot even remember whether you multiply or divide by 12 to convert feet into inches.
I have a similar triangles problem that I give my students every year where the person is shown as 5 feet 9 inches tall. On 95% of the worksheets that are returned, the man is 5.9 feet tall all of a sudden. Unbelievable how much people will overlook the units.
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I see that same error. Or they will have a question that has a mix of measurements and they just use them as is, without making them all the same. They usually don’t even notice, or if they do, they either don’t realize the importance or don’t know how to change it so they don’t.
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