## Thursday, October 22, 2009

### What the Difference an "F" Makes...

So I had one of these moments yesterday when reviewing for a midterm, the kind where you realize that something obvious to you might not be obvious to them. There's a "teachbable moment" there, for me as well as them.

I wrote on the board:

What is the difference between these two phrases?
"25% of the original price"
"25% off the original price"

Forget the math for a moment and tell me in English.

The only difference is "of" and "off". That one letter "f" makes all the difference in the world in solving the problem. And they have all seen commercials for sales on TV. This is "Math in the Real World", which they're always asking about.

If something is 25% "of" the price, we're multiplying by .25. Or even better, we're dividing by 4. (Some of my high school freshmen didn't understand this connection between .25 and a quarter despite the number of times I talk money.)

If something is 25% "off" the price, we're still multiplying by .25, but that isn't the price we're paying. That's the price we're saving. That's the discount from the original price. (I made sure to say "from" instead of repeating "off" there.)