For the people who grew up immersed in the passing over of a Half-crown when asked for "two and six", it wasn't complex thought of any kind. They just rote memorised a dozen or so combinations of coins and the names given to them, and the shops set prices to suit ("two and six" rather than the modern $19.95; thr'pence rather than $2).
The simple divisive properties of base 12 and 60 number systems makes them much easier to memorise than the base 10 stuff we use now, to the point that modern people are probably doing much more mental arithmetic than the folk who used L.s.d. or feet and inches all the time. Of course, easier is better, and that's why we should go back to L.s.d. <cough>
Turn off the GPS if you've got the time (and fuel) for a cruise, grab a pen instead of a calculator next time if you want polish your math skills; but for the most part you should save the time and energy for something more productive.