I would hope by now everybody has figured out when you see a price like $39.99 that it might as well be forty bucks. Dropping that penny from the price is done is to fool the consumer! Since we read numbers left to right, the merchant hopes you will see the 3 and assume the price is emotionally less than 40. This is the concept of “significant digits” which figures in the rounding rules.
This concept of “lying to yourself” can be formalized with different functions and can turn out to be useful in the real world. I am