Anne, you had it easy, doing that in a shop.

My first semi-real job was as barman at our sailing club, aged 17. (I was underage, but it was a private club so that was OK. 17 years ago too, incidentally.) The cash register was an old mechanical monstrosity, and although it claimed to add up totals and work out the change, it simply didn't - its innards were buggered up somehow, so every so often it'd get things wrong. If it was majorly visible then it'd be fine, but generally it'd be a small amount which you wouldn't necessarily notice.

Result - anyone doing the bar had to be able to add up the price in their head, and work out the change in their head too. As Anne and other shop-workers know, there's some pressure to do this quickly. But you really don't know what pressure is until you've been adding up the numbers with two dozen beered-up blokes waiting to get served! :-)

My mum used to use the household accounts as a way of exercising my sister's and my mental arithmetic with a massive long series of "plus £5.29 - plus £29.45 - plus £86.20..." Having two of us gave a cross-check for when one of us got it wrong, and she must have been doing the sums herself to check as well.

"Five-and-twenty" is fairly old useage, isn't it - remember "Four and twenty blackbirds baked in a pie"? German still uses that notation. Since that the rest of the number goes in order from highest to lowest, it's not exactly intuitive, which might be why it's pretty much died out.

Graham.