Rocket Science vs. Coupons

While shopping yesterday for a few grocery items, I noticed a coupon attached to the box of baggies that I was buying. I decided to grab another box with another coupon because I just couldn't pass up a deal. So, now I had two coupons worth 20 cents each for a total of 40 cents off. Not a huge savings but extra money in my pocket is always better. When it came time to check out, I handed the cashier the coupons but they wouldn't scan. So, she continued to try and manually enter the amount to be deducted. But instead of subtracting the 40 cents from my total, she added 40 cents. This could have easily been corrected by taking off the 40 cents that was just put on, and then taking off the 40 cents that I was owed from the coupons. Instead, here is what she did. She took off 20 cents, and then 30 cents, then added 30 cents, subtracted 30 cents again and then gave me my total. Did you follow all of that? So far, she has actually only taken 10 cents off. I insisted that she still needed to take off another 30 cents in order to give me the full 40 cents that I gave her in coupons. By now, there were at least 2 more people in line behind me that really just wanted me to leave. The cashier finally gave me another 30 cents off just to get rid of me, but she still couldn't see why. So the question is, which is harder, rocket science or coupons? What this really teaches me is that kids aren't learning math in school...they are just learning how to push buttons on a calculator or cash register in this case.