Conversation overheard at a local retail store:
Salesperson: "Ok, so for today only this necklace is a further 10% off the 50%-off price."
Customer: "So, why don't you guys just say 60%?"
Salesperson: "Oh, well, we just say it like that."
How about "she doesn't say 60% because IT'S NOT FUCKING SIXTY PERCENT!"
Last time I checked, 1-(1-.5)*(1-.1) = 0.55, or 55% off. This problem is fundamental and appears to be rampant.
I asked a similar question on a quiz in a physics section I had to teach. Simple enough for a college student to get: "If light passes through a lens that transmits 30%, then through another lens that transmits 40%, what fraction of the initial intensity is transmitted in total?" How many people do you think said 30% (actually, I got a fair amount that thought 70% was transmitted...)?
The next day I phrased the problem like this: "If a shirt is 50% off to begin with, and then the teller takes off another 50% because she likes your face, how many of you think the shirt is free?"
I didn't get any hands.
These are college students. Not even stupid, uneducated Verizon employees, but actual UC students.
I weep.