One of the problems on the 6th grade NY State math exam looks like this:
What is the value of the expression below when r = 2 ?
9 – 3r
A 0
B 3
C 6
D 12
My first thought upon reading this was, more or less: “Nine minus six equals three”. So I looked at the four choices to see where the 3 was (in this case, next to B).
But then, afterward, I realized that what had gone through my mind was really more like this:
(1) Because I had just read `r = 2′, the `3r‘ looked to me just like a `6’. I know it doesn’t say `6′ — it says `3r‘ — but I saw a `6′.
(2) Because I saw something that looked to me like `9 – 6′, I just did the subtraction.
I think the take-away here was that I looked for a way to get rid of the variable r as fast as I could. In this case it happened so fast that it was done before I was consciously aware of what was going on. Something deep in my mind was apparently saying: “Danger, variable encountered! Must be eliminated!”
This reminds me of what we do all the time with natural language — something most people encounter far more often than they encounter algebraic expressions. When somebody says to us: “Wear that tie I like,” — and we happen to know that they really like the pink tie — we might actually end up thinking that they had said: “Wear the pink tie.”
In other words, we substitute the variable right away, as fast as we can, going from the abstract description (“that tie I like”) to the concrete result (“the pink tie”). Except in real life, with real objects, we make these sorts of substitutions so easily that we generally don’t even notice that we’re doing it.
Perhaps mathematical reasoning starts with something as simple as learning to repurpose the ways of thinking we all use for natural language (which is an innate ability), so we can apply those ways of thinking to numbers and variables.