I went to buy a book in Waterstones this week. Its recommended retail price was £25, but there was a sticker on the front saying there was £9 off. Woo-hoo!
The girl on the checkout zapped the book. "Oh, the computer hasn't taken the £9 off!" she said, and she walked away.
I watched open-mouthed as the girl returned a minute later with a pocket calculator and began to punch in a calculation.
"It's £16," I said: "twenty-five minus nine is sixteen."
"You're right!" the girl said, clearly impressed. "I'm hopeless at maths." I didn't say that I could tell.
"The trick is to take off ten and add one," I said. The girl looked at me as if I was from another planet. "Taking off ten and adding one is the same as taking off nine, but it's easier," I tried to explain. The girl looked back at me blankly.
So I paid my money and left.
Thinking about it afterwards, I should have pointed out that 9, 16 and 25 represent the squares on the sides on a classic Pythagorean 3, 4, 5 triangle.
That should have made it a lot easier.