Derek Thompson over at the Atlantic shares 11 ways consumers are hopeless at math (via Flowing Data):

You walk into a Starbucks and see two deals for a cup of coffee. The first deal offers 33% extra coffee. The second takes 33% off the regular price. What’s the better deal?

“They’re about equal!” you’d say, if you’re like the students who participated in a new study published in the Journal of Marketing. And you’d be wrong. The deals appear to be equivalent, but in fact, a 33% discount is the same as a 50 percent increase in quantity. Math time: Let’s say the standard coffee is $1 for 3 quarts ($0.33 per quart). The first deal gets you 4 quarts for $1 ($0.25 per quart) and the second gets you 3 quarts for 66 cents ($.22 per quart).

If you’re like me and fascinated by behavioral economics, I suggest you read Predictably Irrational by Dan Ariely.