You got (math) skillz?

I am shocked at the (dis)ability some people have with low-level math skills. This is what I encountered today:

I was at Home Depot and saw that they had Vine Maple trees for 75% off. Normally $20 is now $5 right? Ok.

I took one to the checker and the computer rang it up as $10--only half off, right? So, what did she do? She gave me another 25% off, but that isn't right.

There is a difference between an additional 25% off the original purchase price and 25% off a sale price that is already 50% reduced. She could not wrap her mind around this. I will lay it out below in easy-to-follow math.

$20 (original price) - 75% (or as mathemetician write x0.25) = $5 purchase price

-vs.-

$20 (original price) - 50% computer screwed up price = $10 - an additional 25% = $7.50.

So, a simple math error means I would have paid an additional $2.50 had I not contested it. Seems small no doubt, but when the final, real cost is only $5, an additional $2.50 is an increase in price of 50% because someone can't do math.

I will spare you the 15 minutes of me sitting at the register trying to explain it, the phone calls she made to her manager, and the looks I got as though I was trying to cheat the store.

It all ended with me getting the correct price and her saying, "Don't worry, I'll figure out why it is correct later." [sigh]