I was in a shop the other day and at the till the guy said that there's 15% off, so I went OK that's about £67 then (it was £66 something). He looked at me like I had two heads and said 'wow - how did you do that so fast?'.... errr "I took a tenth off, plus half of that to make 15%...". His reply? "I couldn't work out a tenth..."
The guy was probably about 40.
Please can I hit my head into a brick wall?