A short test
Lady Bracknell, who was early for an appointment, went into a charity shop to look at its book section.
She espied a hand-written sign which said,
"Paperback novels, 75p each or 2 for £1. All other prices as marked."
Having chosen four paperback novels, Lady Bracknell approached the till.
"I'm sorry", she said, "I don't think I have £2 in change".
"Not to worry", replied the assistant. "We have plenty of change today".
Lady Bracknell counted the coins in her possession and discovered that she had £1.92. She advised the assistant of this fact, and paid for her books with a £10 note.
The assistant handed back £8.10 in change.
When Lady Bracknell queried the amount of change she had been given, the assistant explained that the shop's new till didn't recognise amounts ending in zero, so she couldn't key in £2.
What was the mistake the assistant made?