Range - David Epstein Page 0,31
expression for [the cost of] N hot dogs.” The students need to learn what it means for a letter to represent an undetermined number. It is an abstraction they must grasp in order to progress in math, but not a particularly easy one to explain.
Marcus volunteers: “N over three dollars.”
“Not over,” the teacher responds, “because that means divided.” She gives the correct expression: “Three N. Three N means however many I buy I have to pay three dollars for [each], right?” Another student is confused. “Where do you get the N from?” he asks.
“That’s the N number of hot dogs,” the teacher explains. “That’s what I’m using as my variable.” A student named Jen asks if that means you should multiply. “That’s right. So if I got two hot dogs, how much money am I spending?”
Six dollars, Jen answers correctly.
“Three times two. Good, Jen.” Another hand shoots up. “Yes?”
“Can it be any letter?” Michelle wants to know. Yes, it can.
“But isn’t it confusing?” Brandon asks.
It can be any letter at all, the teacher explains. On to part two of today’s lesson: evaluating expressions.
“What I just did with the three dollars for a hot dog was ‘evaluating an expression,’” the teacher explains. She points to “7H” on the board and asks, if you make seven dollars an hour and work two hours this week, how much would you earn? Fourteen, Ryan answers correctly. What about if you worked ten hours? Seventy, Josh says. The teacher can see they’re getting it. Soon, though, it will become clear that they never actually understood the expression, they just figured out to multiply whatever two numbers the teacher said aloud.
“What we just did was we took the number of hours and did what? Michelle?” Multiplied it by seven, Michelle answers. Right, but really what we did, the teacher explains, was put it into the expression where H is. “That’s what evaluating means,” she adds, “substituting a number for a variable.”
But now another girl is confused. “So for the hot-dog thing, would the N be two?” she asks. “Yes. We substituted two for the N,” the teacher replies. “We evaluated that example.” Why, then, the girl wants to know, can’t you just write however many dollars a hot dog costs times two? If N is just two, what sense does it make to write “N” instead of “2”?
The students ask more questions that slowly make clear they have failed to connect the abstraction of a variable to more than a single particular number for any given example. When she tries to move back to a realistic context—“social studies class is three times as long as math”—they are totally lost. “I thought fifth period was the longest?” one chimes in. When the students are asked to turn phrases into variable expressions, they have to start guessing.
“What if I say ‘six less than a number’? Michelle?” the teacher asks.
“Six minus N,” Michelle answers. Incorrect.
Aubrey guesses the only other possibility: “N minus six.” Great.
The kids repeat this form of platoon multiple choice. Watched in real time, it can give the impression that they understand.
“What if I gave you 15 minus B?” the teacher asks the class, telling them to transform that back into words. Multiple-choice time. “Fifteen less than B?” Patrick offers. The teacher does not respond immediately, so he tries something else. “B less than 15.” This time the response is immediate; he nailed it. The pattern repeats. Kim is six inches shorter than her mother. “N minus negative six,” Steve offers. No. “N minus six.” Good. Mike is three years older than Jill. Ryan? “Three X,” he says. No, that would be multiply, wouldn’t it? “Three plus X.” Great.
Marcus has now figured out the surefire way to get to the right answer. His hand shoots up for the next question. Three divided by W. Marcus? “W over three, or three over W,” he answers, covering his bases. Good, three over W, got it.
Despite the teacher’s clever vignettes, it is clear that students do not understand how these numbers and letters might be useful anywhere but on a school worksheet. When she asks where variable expressions might be used in the world, Patrick answers: when you’re trying to figure out math problems. Still, the students have figured out how to get the right answers on their worksheets: shrewdly interrogating their teacher.
She mistakes the multiple-choice game they are mastering for productive exploration. Sometimes, the students team up. In staccato succession: “K over eight,” one offers, “K into eight,” another