Why are petrol prices displayed with, e.g, as 79.9 pence a litre, and not simply 80 pence a litre. And while we're on the subject, why do shops charge, e.g, �9.99 and not a tenner (it's the little things that wind people up, and these are two of mine).
because that is the price and the price changes in increments of 0.1p
It's a psychological thing - people think it's cheaper if in the 9 pound...... range rather than a tenner, though in reality it is merely 1 penny off a tenner. 2ndly the shop is allowed to advertise something as being "less than a tenner" if it's priced �9.99
Another reason for the �9.99 part is that it forces the cashier to go into the till to get your change, making sure that the sale goes through the till and not into the cashiers pocket!