..3 pounds 6 shillings and tuppence and multiply by 8.

Did that in my head with no problems - much easier than similar tricks with decimals. Eight times £3.31 (the nearest I can get to £3 6s 2d) - well, I can do that, but I'd sooner have a pencil and paper to check up, because the columns tend to get muddled up in my head if I'm not careful.

Working out in your head whether you had enough change to buy a random collection of small purchases was actually much easier before decimalisation, believe it or not.