A while back, our senior Welshman, Alan Lloyd, tried to explain the old monetary system that was used in Britain before they adopted their current decimal type system. My eyes glazed over.
Take a gander at Tom Lehrer explantion. Don't know if it makes anymore sense, but it's a hoot.
Ah, decimalisation in 1971: I remember it well.
But as for that conversion, 6/5 or 6s5d (that is shorthand for six shillings and five pence in old or real money) is 6x5p (a shilling is 5 new pence, np) plus a tad less than 2.5p (as sixpence is 2-1/2 new pence) so lets call it 2p (five pence is, obviously, less than six pence, a tanner).
I make that 32p - what is so hard about that? As for guineas: or 21/- in old or real money ...