I have come across this lack of basic mathematical ability twice now in the past month...
First I was at a Department store attempting to buy a curtain that had been reduced in price by 50% (it was £55 originally). The assitant scanned the item and to his horror it did not come up with the reduced price so he was left scratching his head for a couple of minutes before I helpfully pointed out that half of £55 is £27.50 he was literally trying to work it out on his fingers (I thought he was going to take his shoes and socks off!).
Today I went to a book shop with a 15% off voucher - picked up two books and took them to the till. The very pleasant young man serving me got his calculator out and tapped away for a bit before asking me how to work out percentages - I told him to divide by 100 and multiply by 15 to get the percentage to take off my purchase price. He duly did so.
Both were young men in their mid twenties neither seemed in any way to be particularly stupid just completely clueless. Is it too much to ask that people leave school able to do simple calculations - both of these sums I could do in my head and without the aid of paper or calculator so what the fug is going on???