Basic Math

I am so sad sometimes. There are people in this world who do not know basic arithmetic. Then, there are those who get confused with percentages and fractions. All in all, though, I think it is sad that people can go through life without knowing basic math. If you go to a store and want to purchase 3 items that are each $3.00, then, before tax, you should expect to pay $9.00. Sadly, if there was a deal that was "3 items for $10.00", many people would complain when it rings up the items to be $3.00 each.
The site Not Always Right deals with interactions between customers and the people serving them. Most of them deal with people being idiotic, aggressive and outright unpleasant. Some deal with people who cannot deal with basic math. To live in a world with these people? It is disheartening. At least the people who deal with them seem intelligent enough.
20% off $100 isn't $80?
$7.99+$7.99 is greater than $20?
2 for 6.99 or 1 for 3.50 and 1 for 3.49? The first sounds better, right?
4 times (approx.) $3 is (approx.) $12?
3 times $4 is $12?
What is this sorcery? What do these numbers and figures mean? I am so confused.