it would most commonly be 3/6 not 3/9. But it is still easy, 6d is half a shilling and 9d is three quarters. simples.
I grew up with LSD and it really wasn't hard. I was on the till at my uncle's pub at age something like 11 and I never had any problem. And these weren't your put in amount tendered and I'll tell you the change type - it was all in the head.
£11-3-9 is quite a lot of money so 7 of them is a big purchase. What are you buying?
Old money aside, it isn't a laughing matter. There are an awful lot of people in a not good place. There was an artisanal glass blower on the news last night who said that her company in Bath, Bath AquaGlass, which has 17 employees has seen her utility bill go from £14,000 to £233,000. She has said that she simply can't pay those bills.