The antipodes decimalised before we did, of course. Our decimalisation was (i think) in 1971 - might have been '72.

In the days before there was very much automation of arithmetic, the strange number system of £sd meant that many people - even most of 'em - were quite good at mental arithmetic.
Anyone who was dealin with money - especially with change in shops, as I did in my youth - learned a whole load of conversions and tricks.

There were 240 pennies to a pound - that was the base
(if you worked in the garment trade, you got to know that a guinea [£1/1/0 or 1 pound and one shilling] was 252 pence - few other trades used the guinea after the war)
and you soon worked out the number of pence in each shilling:-
2/- = 24d
3/- = 36d etc etc

half-a-crown (2/6) was 30d (this was one of the most useful coins in the range, btw!)

8 half-crowns = £1
(for some reason, 7/6 was known in the fruit and veg trade as "three half-crowns" [pronounced "free 'alf-crahns"]: No other combination of half-crowns was known in this manner)
Half-crowns were also known as "half-a-tosheroon" (no idea why) or "Half-an-Oxford" A crown - five shillings [obsolete post-war] was, as previously noted by dixbie, called "a dollar" and "Oxford Scholar" is the rhyming slang - hence the slang for 2/6.


100 pence is 6/8 (six-and-eightpence)

8/4 is a third of £1.


All of this knowledge was essential if you were going to work out whether you were getting a bargain or being sold a pup. Especially at 5 am in the Veg market!!
I have lost the ability to work out in, my head, how much 1 orange costs from a box of 263 at £1/3/6 the box, but that also was an essential ability in those pre-electronic, pocket-sized calculator days. Oranges came in a variey of sizes, but only one size of orange box, so you got all sorts of different "counts" per box.
But I can, at a pinch, still perform, in my head, fairly simple multiplication and division sums in £sd. (It's much easier to do after some LSD - but the answer is usually wrong.)

Pfranz speaks of his ma wrapping up threepenny-bits in silver foil: The 3d pieces up until the '50s (I think) were silver (known as "Joeys", for some reason!)- tiny little coins of real silver until about 1920-something, then cupro-nickel thereafter, like the sixpenny-piece. My Ma just bunged 'em straight in the mixture - I've still got about half-a-doz of them, stored away!

Y'all out there will never realise the pleasure it has given me to deal with the old currency again! Thanks for the original Q, Jackie