In the days when mental arithmetic was part of daily life, the changeover wasn't that difficult. A florin was still a tenth of a pound, 120+ years after it was first introduced and the continued use of existing 'silver' coins made things quite easy. With the old penny demonetised, there wasn't even the need to use your 2.4 times table. For small change, all you needed was an approximation to know you were in the right ballpark - an ability which people appear to be sadly lacking today, cf. D Abbott MP.